My sympathies to anyone trying to parse Ruby. I tried that 15 years ago and it didn't turn out well. This was, at least at the time, a very well-known issue unique to Ruby compared with pretty much any other popular programming language. There were many high-profile discussions on the ruby language mailing lists on this topic and who was going to maintain the horrendous YACC file (around the time of the Ruby 2 transition). I stopped working with Ruby shortly after so I do not know if this was ever improved (it looks unlikely).
The concern about the breakage of TCA when this flag is enabled is real and concrete. Significant amounts of this thread is devoted to that topic and the core team and project lead have even been weighing in to provide what assurances they can.
This is one of many syntactic migrations for Swift 6. The formal SE process, like all processes, is incomplete and doesn't have a vehicle for every unique consideration. It is my recommendation to the core team to not remove the migration flag until there's a good place for developers to migrate to, and that would (ideally) be language support for case paths. A recommendation is all that proposal authors can give, as a proposal itself is a recommendation to the core team.
The rest of your concerns appear as a vague unease about the changes. This proposal goes to great lengths to explore and try to address this vague unease (a fact that's somehow being used against it).
Do you have any information whatsoever that would clarify your vague unease? If you do, please share it so actual discussion can take place.