University teams may also be less anchored to an individual star since they age out so quickly, and could have a coherent identity for decades under a single coach
I don't think an MLS second tier would be too far behind in terms of quality, and the defacto second tier - the USL Championship - currently has attendance roughly double that of the Scottish second tier[0] which is ample to sustain a professional league, and that'd surely improve if there was the possibility of promotion to the MLS on the line. But if I understand it correctly there's some weird system where the MLS organization itself owns the teams, so presumably they would be resistant to any of them being relegated, unless they also owned those in the second tier.
But in reality promotion and relegation isn't a thing in US professional sport, and it's not structured such that it's really feasible or desirable. That's not necessarily a bad thing, it's just different.
[0] - 5,061/game on average over the 27 USL teams in the regular season (rising to 7,841 in the post-season) vs 2,237/game in the mostly-pro Scottish Championship
It can't happen because that floor is useful to the owners, I imagine if by some miracle it did the NFL would split into two leagues. There's a bunch of non NFL football that could also fit into that lower tier, arena, cfl, fcf.
The NCAA is the other force keeping this from happening. The Sooners saturate the need for an Oklahoma football team, there doesn't need to be a second or third tier team there. But it'd be more interesting and competitive if these leagues were integrated together.
I also would hope promotion/relegation would help with the tanking that's so noticable in the NHL and NBA but don't know enough about drafting/recruiting in European football (and there's probably much simpler solutions there)
Though on the flipside failure is punished and that punishment can be brutal. Teams can enter death spirals on being relegated - important players may have release clauses on their contracts stipulating that they can leave on relegation, and the reduced budget from playing at a lower level could make recruiting replacements harder. Some teams handle it well, while others enter multi-year collapses. So while Norwich City appear to be quite happy bouncing between the top two English leagues, back in 2017 Sunderland were relegated from the English premier league to the Championship and the next season were relegated to League One where they got stuck for a couple of seasons (there's a Netflix series that happened to start around this time called "Sunderland 'til I die" which covers this period, it's really interesting and actually a couple of my friends briefly appear in it by coincidence).
It's maybe more tempting for MLS to focus on money-spinning continental competitions like the CONCACAF equivalent to the Champions League. But I think they'd be missing out if they didn't at least explore a "US FA Cup" as a means of growing interest in the domestic game itself, which indirectly benefits them.
The results are that there isn't much room to grow the sports. Youth development also suffers, teams have no incentive to develop youth talent, and youth teams are generally pay-to-play, meaning it is hard for the poor to join.
I guess if the sports is not popular in US, this may be the only way of survival though
On the other hand, promotion/relegation needs a big enough market to support multiple tiers. Amazingly, the UK has several layers of leagues that attract fans and seem to be economically viable (in some sense) but MLS struggles to attract any attention at all in the U.S. and how second and third tier leagues could be viable is beyond me.