I dont have to answer that because he didnt do it.
He still had the tag on tobi, nothing was stopping him from jumping to tobi anytime he wants and killing him and freeing the world from him, but then he decides to suicide instead, knowing he could have killed him (even though tobi could have done nothing according to you)?
Wow you make minato look like a complete retard for not being smart enough to just jump after him and kill him.
There is no way around it my friend.
Either minato was capable of finishing tobi, which makes him the biggest retard in narutoverse for not doing so, or you can have smart minato planning ahead and deciding to do what he did since he knew thats the only way to stop tobi.
Tobi definitely underestimated minato, thats why he backed off as soon as his current plan got crushed by minato, but THAT is the point.
He can retreat anytime he wants and come back later when he has more power at his disposal (of which he actually talks about in that chapter "i have many means at my disposal).
We now know that he indeed has alot at his disposal, like a spare pair of rinnegan eyes, but they are his longterm plan, probably in nagato´s sockets already during that time.
No one can stop him from teleporting away and waiting for his other longterm plans to come to fruition.
If minato could have teleported after him and finished him off, he should have done so, but he didnt.
You can never play the IF IF IF game and win ("if minato had done that, tobi would be dead" or "if tobi would have done that, minato would be dead"), since the IF game works in both directions.
The only thing you can look at is what actually happened in the manga.
Minato didnt teleport after tobi, which means he atleast wasnt sure whether he would have been able to beat tobi or not, instead he decided to go with the safe way: giving kyuubi to his son.
Any other IF IF IF doesnt matter. Thats how it happened and thats how it was explained.
It was already pretty close to being a plothole in the manga, with minato being able to shut down tobi´s plan but still sacrificing himself, and depending on how you interpret the scene, it either is a plothole or isnt.
I rather go with the non plothole version, since plothole would mean kishimoto messed up and retconned his earlier statement that he made (minato telling naruto that he wasnt able to do anything against tobi).
if you go with the non plothole version, then that statement ("i wasnt able to do anything to him") makes a lot more sense, since you can interpret it as minato realizing that harming tobi´s fake body (which it was, he just throws his arm away again, just like against the insects) is nowhere close to stopping tobi´s agenda.
Minato realizes that tobi isnt someone ordinary you can just kill off, he managed to connect a full rasengan and stabbed him in the chest, and it didnt do shit to tobi, tobi just went on and threw his arm away like it didnt even matter (which we know is true, since he can replace them), and told minato that he is the ruler of the world.
Minato believed him and looked troubled, and thats the moment he decided to do what he did.
Thats why we have a small flashback to exactly that scene, shortly before he summons the shinigami, making the viewers understand why he does what he is about to do.
What kishimoto did was basically making kushina say what the viewers would say. "Why minato, i wanted you to be a father to our son, why sacrifice yourself now" and that is the moment where he repeats that tobi wasnt lying when he said that he was the ruler of the world.
Its all written down in the manga, and minato is dead because of that explanation.
Credit where credit is due though, minato definitely was one step ahead of tobi, since he managed to create the jinchuuriki naruto.
I could be nitpicky and ask you the opposite "why didnt minato just teleport after tobi and kill him after he escaped the scene?" but as i said, the IF game leads nowhere.
The only thing we can look at is what actually happened in the manga, and in the manga, minato didnt do it, which suggests he wouldnt have been able to kill tobi just like that.