While I understand your point and I agree and think the same way, there is one small point, which should not be underrated in this: "a VPN is just another ISP" is only true as long as your ISP does NAT and you don't have a publicly available IP. That is true for some mobile networks here in Germany for example.

So a VPN (or things like TOR) provide you with one single out-point/edge/end-point that is (hopefully) shared with many many other people. With that you can hide in the crowed and it is very difficult to find you in the crowed.

On the other side it would defeat the purpose if the VPN provider limits the number of accounts on one out-point/edge/end-point to improve performance, because then it would defeat the purpose.