While HGL is an open league, I think it's good to keep in mind that being selected to play in HGL is a priviledge, and not a right. :)
Why is it a privilege?
Since when do 'we' want to limit the growth of 'our' HGL?
There is no need for you to rev up or start a round of accusations (on your another post) due to a friendly reminder, that it's always good/wise to not take things for granted.
It's also good to remember, that sometimes it's a strange thing with human nature, that some people value things more if they have paid for it (as paying kind of forces them to justify the importance of something, as they have made a decision to pay for something and then it often makes them appreciate it more, and enforce the thinking that it must be important as I've made decision to pay for it), so something being free sometimes causes whatever-kind of attitudes. So far, as we know, we haven't had issues with that, of which I'm thankful of.
(And yes, while 100% obvious, I'm not suggesting there should be any price in participating to HGL as the main spirit/idea of establishing HGL is to make it open and free to all qualifyiing participants - yes, given current/possible league restrictions in terms of expanding during ongoing season at any given season, (such as current league size "restriction" of max.18ish to 20ish teams).
It was also to say that most often it's not optimal to rush things, as always everything affects everything and most often if you rush to reach any decisions, you'll end up seeing that it should´ve been given a better thought (to reach more optimal solution). In this case, there isn't 100% easy solution to fix all issues without causing any downsides, as we well know.
I have also presented a perfectly functionable models (or rather a prototype at this point) on how to decide how teams participation rights for next season of HGL could be decided so in that sense if you want, you could consider that "play-in league/play-in playoffs" as a lower level division, Division 1 (due to possible, but definitely not decided transformation period into two different league levels) as it would ultimately be a decisive "league" that decides participation rights to HGL (while technically as it would likely include only few teams, it wouldn't include that many games).
It is good idea to try to gather many teams to share this HG journey in form of participating to HGL, while simultaneously we need to remember that - like you mentioned - it can't expand as it is without any limits (as number of matches per regular season would rise dramatically). This is also why I presented my clear view (by giving example from my "regular BB league") on why dividing teams to two same league level groups has large downsides, just as dividing teams to two different level leagues has.
However, this wouldn't suggest that it is an automatic right to be able to participate to HGL by the similar type of meaning as having human rights (or something like that). Quality is always more important thing than a quantity.
As HGL will remain a manual league till the unforseeable future (unless BB devs would want to make it as an official part of BB, which I kind of suspect they wouldn't as communication towards users have been almost non-existent to my knowledge), the quality of commitment from managers (or a lack of thereof) can sink the league (or at least cause trouble to Commissioner and cause penalties) if there would be managers that don't want to comply with the league practices/rules. And no, the proper solution is not to throw the rules to the bin. HGL should never be "a wild jungle with according laws".
But this is a complex issue all in all, due to which it will be necessary to sink the thinking cap deeper onto the head. Luckily though, immediate decision isn't required to solve anything, as there are currently no more than 17-18 participating teams to Season 3.