Well, there is an achievement for getting to L4. There might also be a point to it in the Solo Eternal League. At least one competition requires a Level 2 Stadium, to represent a rich team, so it's possible others might need L3 or L4.
Always possible that Cyanide will add more stuff.
They should have used rules from the BB2016/DZ for stadiums, those can be applied without the rest of new rules pretty well. And Cyanide's sponsors are cosmetics as well, though new rules have an interesting implementation of sponsors too.
Don t be so negative. I beleive they will listen to the community and bring more modification to their rules and enhancements as they go . We just need to be patient . I also beleive nothing will change drasticaly till the new rules get tested intensively and some kind of general consensus in the community comes to light about what change is acceptable or not.
Saying that something should be done this or that is not being negative, it's being constructive. And they seem anyway use the new rules as "source of inspiration" for their own thing, I very much doubt it was coincidence they came up with Sponsors and Stadiums around the same time GW did :) They just didn't finish their job. And it's not just they did it differently, mechanics of Stadiums in the new rules are significantly more rich and interesting, it provides you more options and don't affect balance much more than Cyanide's current implementation.