If they weren't content, they would move to other games - maybe not other ttrpgs, but other genres entirely.
But people stick around. So subclasses as a system aren't ruinous.
This is a fallacious syllogism. The inferences being made here do not logically follow. It's once again unsound reasoning. I am fine with you arguing a general opinion that most people are likely content with 5e subclasses. I would agree with that. My problem, however, lies with your argumentative reasoning that surrounds that point. It is reducing a complex choice composed of many constituent parts (i.e., reasons to choose a TTRPG) to a sign of satisfaction with a single factor (i.e., subclasses). We could basically substitute any given mechanic or aspect of 5e into where you have "subclasses" above, and the argument would be just as fallacious.
There are a variety of reasons why people are primarily sticking with 5e D&D that has nothing to do with their overall satisfaction with subclasses. They may be staying despite their negative opinions on subclasses. They may like subclasses or they may prefer other class structures, but they may not be moving to these other systems because they dislike other game mechanics involved in those games. For example, I think that DCC has too many polyhedrals, and I loathe magical physical corruption, magical mishap, and character funnels. So DCC may not have subclasses, but subclasses aren't pertinent to my decision-making process for why I don't play DCC.
You may then conclude that this means that subclasses "aren't ruinous." But let's understand that there is a lot of space for nuance and complex opinions between setting a low bar of "aren't ruinous" and "I love 5e subclasses as they are!"
For example, some of past players that I have gamed with didn't like that there was very little decision-making or character building when it came to subclasses. They picked their subclass, and that was essentially the last decision they made for building their character. (I am fully aware that this is also a feature and not a flaw for some players.) Some of these players tended to gravitate then to options that were more open to building your own character as you leveled: i.e., the Warlock. If you made all classes structured more like the Warlock, these players would absolutely love it! 5e style subclasses "aren't ruinous" for these players, but I can also tell you that they weren't all that happy with them either.