I certainly would not argue MLS is currently as strong as the top European leagues (i.e. UK, Spain, Italy, Germany). The reason why, however, is not because it is filled with a bunch of used up has-beens. Rather, it is because the top young talent typically leaves for EU in search of money and bigger stages.
The good news for MLS however is that the Top EU leagues are now paying large transfer fees to the MLS clubs for this talent, which if invested wisely back into the clubs, will improve the overall strength/quality of the league. It will take time to get to the level of the top leagues, and may ultimately never get there, but I think it is the right pathway.
Some argue that bringing Messi in ultimately hurts the league by reinforcing the narrative that this is a retirement league. While I don't necessarily disagree with this, I also do not think you can pass up the publicity that the most popular athlete on the planet will bring to your league. Either way, it is a big risk for MLS- if people watching take away that it is a vastly inferior product, it will haunt them well past the days when Messi is long gone. (My guess is that the casual fan turning up to watch Messi will not know the difference.)
BTW, shout out to my West Ham club that won the the UEFA Conference League last night over Italian side Fiorentina. One of the highlights of life as a sports fan. COYI!