I think a big part of this question is how to reach the silver and high-steel team leaders. From my (limited) experience of ugc, the team leaders tend to be the most motivated and focused on improvement, and they are the type of people who would be, in theory, willing to put their team in a league they would improve faster in.
However, despite open being just that, open, the majority of them are scared to register their teams in the division. They (mostly rightly) believe that they aren't "ready" to play in esea. When I first joined an esea team through the S23 newbie mix team drive, I still had a bunch of friends that didn't think they were good enough yet, who were better than me. Was I good? No. But I was ready.
If we can reach out to those team leaders, and somehow get at least 4 or 5 to sign up with their teams, that combined with each season's bottom open teams would in theory give them plenty of competitive matches. I think if we managed to do this, it's our responsibility not to leave them hanging out to dry. Support them, offer mentorship for teams and players, and keep as many of their teams alive for as long of the season as possible so that they can keep having competitive matches.
I don't think it's impossible to accomplish this. I think that an important distinction should be made about playing in ESEA Open. Sell the experience not as "the teams are so much better you'll improve a lot more" but as "the teams and players are a lot more motivated and you'll improve a lot more and it will feel a lot more rewarding". Be realistic, don't tell them they'll be 8-8 in their first season. But we could say "I'm also talking to these UGC team leaders, two of them have already decided to play esea". Make it clear that they will probably start the season against some tough teams, but will quickly start to match against other teams who don't start well- especially other ex-UGC teams and teams that would normally be bottom open.
Some of their players might have problems with paying, so this is where season-long premium codes come in. In my opinion, these would be very justifiable to use, because we would be actively bringing in whole teams to ESEA who wouldn't otherwise be playing in the league.
If we combine all this, I think we could convince at least 4 or 5 UGC teams from steel and silver to play in ESEA. Just imagine how much healthier mid and low open would be with 8 more teams spread throughout the lower end of the league- fewer forfeits at the end of the season, more fun games. Not every player would return the next season, but the ones who truly enjoy the competitive environment are always the ones who come back for a second season anyway.