Pro/rel has some other virtues (as well as other costs) but proper sorting isn't one of them. You start from the assumption that (unless the team is playing at the highest levels) if they are winning 80% of their games (and therefore clearly promotable) or losing 80% of their games (and therefore clearly relegable) they aren't playing at the right bracket and have lost a year being misplaced. Yet if you look at the coast bronze, silver and even silver elite brackets over the years, we see that there are teams clearly at the top of the bracket and at the bottom. Ideally, with properly sorted brackets, the titles are decided by single goals with most teams spread out 50/50 among wins and losses.
My son's first team was a coast team that was required to play bronze even though as a newly formed team they were winning or placing at most of their summer tournaments. They went on to win every game in league, usually by margins of 5 or more, sometimes double digits. I think they only had one game that was even close. But when they got to league cup, and state cup, the players, especially those on defense, were in for a rude awakening when they started battling silver and silver elite teams, for which bronze had done nothing to make them ready.
The issue with pro/rel is that it is more about player recruitment than actually developing the players. If a team gets promoted, it gets access to a higher quality level of player that doesn't want to play "only bronze" or "only silver". They can therefore cut the weaker players and upgrade their existing players, but it's a process which leads to a wasted year for development of the team in the wrong bracket. The individual players, particularly at the younger ages, are also improperly sorted because you get those "impact players" everyone wants and is advertising for which are placed at a team that is lower than their skill level in order to make a difference in wins/losses. With relegation, it could cause the explosion of the team, and a minimum players will leave, which means how they do the following year depends on the coach's ability to hold the team together rather than any coaching (different skill set). The only way to fix this is to say players are tied to their starting team and teams can only upgrade by recruiting new non league players or by trading existing players, but that will never work because the parents of kids that want to move will just move onto another league that doesn't do that.
I'm not saying that SoCal has it right either, just that pro/rel is not the panacea to fixing the issue. The only way around it would be like some European countries do it (and AYSO kind of does it except they do it with equality in mind) which is to assign rankings to kids and have the ranked kids only get to pick among eligible teams they are from (you can bid up, but not down). That would require though a unified national league at all skill levels, and the parents would loudly complain their kids were not assigned the right player ranking (much as happens in Fifa).