I personally think Yama was stronger than all of them, although maybe, maybe one of them is stronger than even he was. But yeah, none of them actually has to be as strong as him for the five of them to outweigh the Gotei 13 in power.
But as for the issue of how there could be a war if they're so strong, it depends on a couple of things. For one, what each RGs power is. Yama wasn't just fearsome because of his strength, but because of Ryujin Jakka. It has tremendous destructive power, and moreover it's highly versatile. It's the type of weapon that can take down a dozen powerful enemies with one AoE attack, as opposed to something like Soifon's Shikai. The RG could be as strong as him, but possess a power that makes it easier for them to be taken down by multiple enemies working together. Also, we don't know what Yhwach's plans are for dealing with the RG. Now that he's got Zanka No Tachi on top of his already formidable power, I reckon he could take any one of them on. In fact, given Zanka No Tachi's range of powers, I don't see how they could take him even working in a group of 2 or 3, but again that depends on their own abilities. Then there are the War Potentials. Of the known ones, Aizen and current-Zaraki could probably give someone of Yama's calibre serious trouble with a team of Sternritter backing them.
So, not saying they're all actually that strong, but a conflict could still work if they were.
Imagine a person stronger than Yamaji??? Then say there were 4 more of them???
Think about it hard and deep.... Then tell me if you thin Kubo can afford making such characters......???
Yama going full Yama can already destroy the human world or karakura town and during the Vandenreich invasion, the captains were hoping that he can finish the fight soon since it would also destroy the soul society.....
If there were 5 people like that??? If they go full retard on the battlefield, it would end bleachverse...
Dunno if that was addressed to me, but again, it depends on what their powers are. Aizen was at least comparable to Yama's full power, but his Zanpakuto didn't have the sheer destructive force of Ryujin Jakka. In fact, both Aizen and Ichigo reached a level above and beyond Yama, and they both managed to go all out without the world ending. Sure, there was tons of collateral damage, but that's nothing unusual. The same could apply to the RG. Assuming they are stronger than him, which I don't even believe to be the case. Potentially one of them is, which still wouldn't necessarily mean he has such a destructive Bankai.
All of them being stronger than him would massively ruin the balance of power, since Yhwach would be the only one capable of fighting them in that case, so that's almost certainly not the case. Unless some other power is added to the mix.
Anyway, random idea, but what if the reason the RG are so strong is that they undergo a ritual similar to what Yhwach does with his Sternritter, that unleashes their full potential? The thing with Uryu drinking his blood (I think that's what was happening anyway). The ritual could give them the power of the Ouken as well as their massive strength. Maybe part of the reason Yama wasn't inducted into the RG is that his already ridiculous power plus a boost would make him too dangerous.
That is what i mean't no one wil be left alive all people in bleachverse will die.. I like the argument that Yama didn't need powerboost anymore though.. that sounds sound...
So does this mean what with the ouken in their bones they are now stronger than yamamoto?
But like I said, both Aizen and Ichigo acquired a level of power surpassing Yama's, it just wasn't as inherently destructive in nature. His world-ending capabilities all stemmed from Ryujin Jakka, not to diminish his strength in general. When RJ got sealed, he wasn't any weaker, he still took out an Espada-class with his bare hands, but he lost the power to wipe Fake Karakura off the face of the Earth.
Aizen was stronger than any Captain, but his Shikai wasn't the kind of thing that could level a city. The same can go for the RG.
I think one royal guard is stronger then yamamoto, and 2 of them are about 70% of yamamoto's power and the other two are above average captains like shunsui which is like 50% of yamamoto's power.
First, no captains were forced to use their bankai. The four captains who lost their bankai did not use it because they had to. They employed it to draw out the enemy, to test how they were able to seal bankai. Unfortunately, it wasn't a seal, they stole the bankais.
But here is Byakuya/Komamura/Hitsugaya's explanation...
Here is proof that at least Byakuya was capable of holding his own, for some amount of time, in shikai
He lost badly because he went into bankai and then was defeated with his own power. As Nodt did not use his own powers, and it is shown that Byakuya does overcome his 'fear' attack and is able to strike at him. However, As Nodt just then uses the stolen bankai to destroy Byakuya. I would argue that if Byakuya had stayed in shikai, he could have at least held his own against As Nodt, although he would probably have needed Renji's help too.
Here is Byakuya breaking free of the 'fear' attack
Point two is that Aizen might have been close to old man Yama in ability, but the fact that he went to the effort of modifying Wonderweiss, just to counter Ryuujin Jakka suggests that he wasn't close enough to think he could defeat him. In fact, he recognizes aloud that he might not be able to overcome Ryuujin Jakka here...
And Finally, Yamamoto wasn't forced to use his bankai to fight the Bach fake. He chose to use bankai before even being challenged. Add to that the fact that the Bach fake only fought defensively, and it becomes evident that had Yamamoto NOT gone bankai, he would not have worn himself down. But he was convinced he was fighting Bach and used his full arsenal. Sure, the Bach fake survived for a while, but his abilities were pushed to the brink and once the full bankai was released, the fake was trashed in one hit. That wasn't a weak Sternritter fighting at Yama-jii's level, it was purely defensive posturing.
Thus, I am convinced that old man Yama certainly would have been able to take on one or two RG members and give them a really good fight.
A Shinigami's power is that of their Bankai as much as it is their normal abilities. Yama's Bankai is the strongest we have seen and it's not even close. There is a reason he told Hisagi he would kill all the Quincies when they invaded, because it's something he is capable of.
It should be noted that Aizen's power only truly became too much for the Gotei 13 once he had the Hōgyoku in him. His powers as a Shinigami were not enough for him to fight all of the Gotei 13 at once, that is absurd. He never even faced all the upper tier captains at one time, and even in the last fight, hardly anyone had their Bankai released.
Ichigo for intents and purposes isn't a Shinigami.
So to me, there is no debate that Yama is the strongest Shinigami ever. He didn't have to use something like the Hōgyoku to achieve massive results, nor is he a super hybrid like Ichigo.