Why do you assume he could fight for weeks? People like to make the argument that gojo could maintain Infinity for two or three days straight when he was younger. But keep in mind this was when he was in a casual state. That is not the same as being able to fight while maintaining Infinity for quite possibly days against trillions of opponents.
Gojo keeping Limitless on for three days straight when he was younger was not when he was in a casual state. Gojo was in a pretty high-stress situation, facing constant fights while trying to protect Riko, was sleep-deprived, and, unlike current Gojo, he had to
manually keep Limitless up (which he did, even while sleeping), and it was far more taxing, Cursed Energy-wise.
Current Gojo is a lot more experienced, so not only is Limitless always up automatically, but thanks to mastering his Six Eyes and Limitless, Limitless's Cursed Energy cost is described as "infinitely close to zero" (or something like that), so it's not going to be overtaxing him any time soon.
Even if Black Sperm chooses to conglomerate, Golden Sperm and Platinum Sperm are restricted, so he could only merge with a lesser number of clones. Once he experiences Infinite Void and Infinity, he'd realize that merging would only decrease his cell stock, so he'd stick to wearing him down.
Black Spermatozoon still has to conglomerate themselves to have any hope of hurting Gojo (remember, just one is so weak a puppy could beat it up), so that will reduce the amount Gojo has to kill.
People really underestimate how many clones of himself Black Sperm has. A billion opponents by themselves is crazy to imagine. Repeat those billion opponents 1,000 times, and you get 1 trillion. Then multiply 1 trillion by 54, and that's how many clones Black Sperm has. Even if all the Black Sperm clones have unknown durability, it's naive to assume that all the clones would be destroyed by Gojo's Red or Hollow Purple.
While it is a lot, Gojo can spam Red and Blue pretty well, so, while it might take a while, Gojo will eventually kill them all.
Gojo would have to use Infinite Void multiple times to really put down the Black Sperm clones. Even for Gojo, he could only use it four to five times against Sukuna before his brain was overloaded due to repairing it with RCT. Assuming it wouldn't take as much stress to activate Infinite Void since there's no opposing domain to fight against, he could probably use it ten times at most? I really don't know, but you really can't say he could spam his domain dozens of times. Even for Gojo, that is a stretch. Considering the size of the domain and the number of clones, it's naive to assume the Infinite Void would be able to kill billions of clones with every domain expansion.
Gojo likely wouldn't use Unlimited Void (or use it multiple times) because he doesn't really need to.
Though, if he wanted to make sure he gets a large amount of Black Spermatozoon with one Unlimited Void, he can just Blue to gravitationally attract a large number of them into a single area.
So no, this is not a stomp, despite what people claim.
I disagree.