I would argue that it isn't that they aren't required to get better - Just that most people are simply too lazy to do such. Or because of disability or just outright hitting their peak sooner than others.

The problem with such measures is that people who don't want to learn - Will never learn regardless of what you try and propose for it, in grouped content, to a certain degree this will just inconvenience the group at large as they will be forced to then pickup the slack.

Further to this though... You need to consider that it is also done with accessibility in mind, e.g., those that may have some form of disability or just something imposed by older age, I don't think people should be relegated to a solo experience with the story just because you don't feel like the game is challenging enough. But one thing to consider though is when you do the upper end of content, e.g., extreme and savage then the remainder of the game on normal level just feels slow by comparison, but if you only do story then to a degree it does actually have progression with difficulty. That said, I do personally think the game has gotten more mechanically challenging, but stuff like gear creeping renders a lot of it moot, similarly, and by this, you just find that things don't hit maybe as hard as they ought to.

I'm not entirely against it by any stretch, but there are implications behind some of the suggestions e.g., why should people who might be bad at the game due to external factors need to be relegated to a solo experience? All this does is goes from punishing one group of players to punishing another group of players, and many of the latter may be of no fault of their own.

Similarly, the next implication is that by enforcing challenging group content that you will force everyone to get better at the game - This is not always the case. Many people hit their peak sooner than others, this is not an appropriate countermeasure for laziness, etc.,