Pro/Rel was great 100 years ago when the business side of the sport was much less relevant. It's such a stupid concept in the modern game, especially with an established top-division.
Arguments for Pro/Rel:
1. It's entertaining and makes the whole season relevant for more teams.
True, but in MLS we have the playoffs, which this season no team was officially eliminated from until week 29 I think. It's not needed from an entertainment perspective.
2. It's a chance for players to move up from the lower ranks to the top flight.
Sure, but it also stunts development of young players as clubs with a desire to move up will hold onto their best players with the hope of moving up, which may or not happen. In these cases, players wind up in D2/3 for much longer than they should when they could be laying in D1. The affiliate system imo is much better for player development, as you can move players up, and down (for playing time) much more freely between affiliated clubs along the divisions.
3. Great minor-league clubs can't get to MLS.
Who cares. They can/could have been in MLS if they wanted to. I think it's much better to bring teams into the league based upon a viable business structure instead of a poorly run/funded team being fortunate enough to put together some good results over just 1 season to get into MLS, where their operational struggles get exposed.
Pro/Rel coming to MLS/US Soccer is a stupid discussion.
I'm not really certain its best here for the US, but to say it's a stupid discussion seems somewhat silly.
I don't know if it will ever work here, or if it really should be tried here, and if so, it really wouldn't or shouldn't be for another 20 years or so while the popularity of soccer here in the states continues to rise.
The added benefits pro/rel would bring would be increased TV markets. At some point, MLS is going to reach a max number of teams that they won't expand from, and being in the US, there will be a lot of untapped markets. Especially considering some of the markets currently in play aren't the largest (Columbus, KC, possibilities of Sacramento, etc).
I'm not certain that other markets will continue to draw fan attention without the possibility of reaching division 1 (MLS).
So the big question to me is, what is the number of teams that MLS will max out on, and what will be left out? Next year we'll be looking at 22 teams with the potential list of teams to come in:
Indy
Cincinnati
Charlotte
Miami
Sacramento
San Diego
Ottawa
I'm sure there are others I'm missing here from the top of my head
That still leaves many US and Canadian markets untapped (Pittsburgh, Tampa, Milwaukee, Tennessee, San Antonio, Oklahoma City, St Louis, San Francisco, other Canadian cities, etc).
The obvious biggest hurdle to getting there (whether it makes sense or not) is there isn't a whole ton of incentive to current owners to make this move. But I think there could potentially be in 20-30 years, if the popularity of this sport was to reach close to European levels. Fans in Chicago may be more willing to show up and watch their team play at the end of a dumpster fire (pun intended) to support not getting relegated. On the flip side, I may actually turn on and watch St Louis vs Milwaukee to see a promotion battle.
Ultimately, when it comes to sports, money talks, and for this to happen, there would need to be viable and fairly certain financial incentives.
I'm not an advocate for pro/rel, but I think there are some very long term considerations. Ultimately though, I just want this league to succeed, and grow to the point where top flight talent comes to the MLS, not to collect a paycheck or as a stepping stone to another league, but because this is the place to play against top talent.