Let's rewind a bit though. The 'players can leave every year' is more of a soccer thing than a MLS thing. Players demand and receive transfers way more in soccer than any other sport in general. If Lionel Messi wanted to leave Barcelona tomorrow he'd have his agent run to the press and then he'd be gone the first day of the transfer window. If Sidney Crosby wants out of Pittsburgh he has to do it super-privately or everyone just laughs at him. Totally different mentality.
Is MLS a stop gap league? I wouldn't exactly call it that. Its obviously not a top tier, or probably even a second tier league. But its not a retirement league anymore, and it is now attracting a good calibre of 24-27 year old player who previously would have never considered MLS. Guys like Giovinco/Lodeiro/Diaz/Romero would be elite players in second tier leagues and are choosing MLS.
The cap stuff is pretty trash though. No one even knows how close any team is to the cap because no one releases how much GAM/TAM/BLAM each team has after trades. MLS just makes up rules to get good players in the league (not a terrible thing IMO, although they once did block TFC from getting a DP because they felt he wasn't DP'ish enough). MLS will never make the jump to the next level until they ditch or significantly increase the cap. Unfortunately they seem to be more focused on getting $100M expansion fees than having a good core group of owners that are willing to spend $25-30M on their rosters.