Base MS used to be 300-330 and boots gave +50. Boots were pretty much a requirement then, as not having them mean you were a lot slower.
Sometime after that, you typically bought them sometime after your first medium-item like BF Sword.
These days, I'm seeing even T1 bought as late as after the first major item and part of the 2nd item is completed (eg. ADC buys IE and zeal before getting boots). Put simply, boots are naturally becoming built later and later.
Aside from that though, making boots entirely optional won't increase build variety at all, and if you make boots any weaker, then no one will buy them. Boots 1 right now are nice to have but not something you actively rush unless you can afford to delay your build a slight bit, and Boots 2 cost enough that the +20 MS they provide better be worth it because the enemy is going to have a 2nd big-ticket item built while you're stuck at 1 and a half.