Natural "extension" of the OPT program is the H1B... It's even in the rules, where your "OPT visa" (F-1) is on purpose not dual intent - you need H1B if you want to apply for a green card. And that's how most students do it: F1 -> OPT (w/ extension if able) -> H1B -> GC

Increasing the H1B threshold makes the next step very expensive, and it will cause more companies to be unwilling to hire w/OPT, therefore reducing the attractiveness of top US schools to the best candidates.

Many countries (that I am familiar with) have a very straightforward path to residency after completing education in the country. US makes it on purpose very difficult. And one problem is, in the eyes of the "system", a degree from Stanford/Harvard/MIT is equivalent to a degree from any other school capable of issuing the "same" degree.