Relegation/Promotion is great and it works in Europe, but that model wouldn't work in the USA... yet. The league is set-up in a way to where it can't fold like it did before.
It's the same way with the salary cap. MLS teams don't get a huge injection of TV money like PL clubs do. If the MLS didn't have a salary cap then the richer teams would just keep on getting better with no limits on spending. At least in the PL the less financially fortunate teams have TV money to fall back on.
The real problem for the MLS (and American players in general) is a lack of a strong grassroots footballing culture, including a poor youth set-up that costs a ton of money if you really won't your child to progess. The better your kid then the more you have to pay it seems. It should be the other way around!
As far as the draft goes... I'm not really sure what you would do for those players coming out of college. It's obviously not the same as in Europe because teams here don't generally have a youth academy and players are coming from all over the USA... it's not as localized like it is across the pond. You could allow them to sign for any team, but then again the teams with the most money are going to get the best players year in, year out.
I just don't think it's as easy as saying "Okay, that works over there. Let's do the exact same thing then." The blueprints for one league aren't necessarily applicable for another league.
There does need to be a lot of change though, and I believe it will come over time. The quality of players keep getting better and better and financial restrictions are getting looser as well. My main gripe at them moment is how much control that MLS, the entity, has on transfers. If a player gets an offer from somewhere else then it has to be negotiated with the team AND MLS. That really bothers me since it raises the price on young American players and keeps them from moving abroad. If Pulisic was in MLS instead of the Bundesliga he'd probably be priced out of a move.