Did you read my post concerning this? Please explain to me how you know beyond certainty that Luffy will face Big Mom before meeting any other enemies, or that there's no way she won't turn out to be Luffy's friend. As Uriel has said, circumstances played a big role. If it wasn't, Luffy would have been fried by Enel no contest.
Fried by Enel ? The guy was Enel's weakness, how can he be fried by him ?
Ok then, suppose for a second that he will not face BM and will turn to be buddies (then why did Oda bother to make Luffy declare war on her, but nvm that), who would he face ? Pirates like Squardo ? No way, obviously too weak for Luffy, Kaidou ? How is that gonna happen since he wasn't even introduced yet ? BB, nope too early, Shanks ? They wouldn't fight and it is too early if Oda ever intend to make them fight, Akainu... Kizaru ? Too early to face an admiral for sure.
Then give me a single character that would give Luffy trouble, but more than that, to make the balance shift again, we all know what Crocodile's defeat has caused. So who do you think would fill that part hmm ? I don't see anyone here besides BM for now.
Again, what kind of enemies will Luffy face ? Allied crews ? You think that a shitty pirate from NW can stand to Luffy atm ? We have all seen how " remarkable " they were in the war isn't it ? It even took 3 captains from the NW to take down a single Pacifista. You think that BM's allies will be stronger than WB's allies ? I doubt that...
But you wouldn't say Crocodile was in the same level of Luffy or that admirals were one more weak than the others, right? Because you just can't measure that of that situation. OP is usually the best one when it comes to screw up power levels. That's why I think that when you say that it's impossible for BM to be in the same league of the other Yonkous it's pretentious as hell because you don't know the circumstances, her ability and how Luffy will confront her.
The war.
Before the war, we have never seen top dogs fighting, we didn't have a single idea about that. Some people (even me at some point) believed that Admirals are only strong thanks to their Logia and anyone with Haki can kick their sorry asses. The war proved us wrong. Let's check what happenned : Aokiji & Kizaru & Akainu unscathed (I'm sorry, even after taking that hit from WB, Akainu was still okay and was pursuing Luffy and dealt with the WB pirates), Garp & Sengoku unscathed (considering those fives as the top dogs of the marines), let's take a look at the WB pirates : WB dead, Ace dead, Marco got his ass kicked, Jozu frozen, Curiel defeated by Akainu. It was painfully obvious to us the complete supremacy of the admirals, and were truly fit to be called " The greatest military power " of the WG. WB was sick, that's correct, but still, not managing to overcome Aokiji's haki when he tried to stab him, not managing to land a hit on Kizaru and Aokiji, and only hitting for once Akainu and not even being able to definitely knock him out.
We got a solid evidence about the power of the top dogs at the war, therefore we can absolutely say that Crocodile is weaker than Admirals and some Shichibukai, and thank god it was the case, or else Luffy wouldn't have gotten trashed that badly in the war. If Croco was as strong as an Admiral, and then Luffy in the war getting trashed, would it make sense ?
The war has showed us what top dogs are worth, and even Luffy with his current strenght still isn't there yet. Before we didn't have that occasion to measure that, but with the war as a reference, it is possible.