I don't think it's pro/rel for pro/rel's sake. It's almost like there's two different viewpoints on this and they may or may not ever meet. I'm pro-relegation and pro-Supporters Shield, but you're probably pro-playoffs. Nothing wrong with either position, it's just that they're different and likely not compatible.
Actually, I would prefer a pro/reg system with full open access...
For me, the beauty of pro/rel is that winning matters for the whole season. With playoffs winning or losing makes no difference as long as you get that last playoff spot. You could totally suck and still make the playoffs, and from there it's only a few wins to the championship. To me that cheapens the whole thing. With playoffs you could lose every game and still keep your spot in the league. But with pro/rel you maybe have to fight for your life to stay up. So instead of everything sucking for the whole season there's excitement to the very end. And it's even better in the league below, as every single point matters as the league winner gets promoted. (That's why the NASL doesn't matter at all, because there's no hope of getting promoted into importance.)
So there's drama at the bottom of the top league and matching drama at the top of the bottom league. And there's also drama at the top of the top league with pro/rel as well. With playoffs it doesn't matter who wins the league at all as long as you're in. No real benefit to winning, and to me that's a shame.
Not trying to convince you or change your mind on this. I'm just pro two leagues and pro pro/rel.
...so I agree with all of this.
I just think the two leagues format is forced and incredibly unrealistic. If MLS 2 is kept so financially viable that teams would agree to it, why would teams care about being relegated? You can't have it both ways.
And as good as the narrative sounds, adding pro/reg only really effects the three-ish teams at the bottom of the first division. The other 17 teams aren't going anywhere, and at the top, teams are already playing their heart out for the title.
Like it or not, we are a large, closed league. Smaller, regional conferences (8-12 teams) with a postseason to determine the national title just makes more sense.