They give an opportunity for lots of players to do something in one location, with good rewards to encourage participation.
I'll admit I skimmed the points so may have missed the nuance about why "harder is better" or why "the way it is now is worse than before", but if it's just "because it used to be difficult to achieve and now it's not difficult to achieve", I'm unconvinced the current scenario contradicts, in any way, your statement in point 4.
I see plenty of community in most CGs I've taken part in recently. I don't think it would be bad if the goals were made tougher, but I don't follow your conclusion and I don't think it supports your premise. CGs aren't the way you prefer them to be, sure. But, I'd easily counter the idea that they were somehow better for community building by my own anecdotal evidence and I'm unsure anyone would be convinced by either of us. I've not exactly studied the numbers but my guess is participation right now is probably pretty good, right? Perhaps there are fewer participating now, or perhaps even if there are more, there may be more who just "hand in once and done". And maybe there's evidence fore that in the numbers, but you didn't present that.
I've had a lot of fun in the CGs since playing recently again. I think suggestions as to how stuff can be better are important. But I'm unsure "it's worse now, it was better then" is the solution, nor am I convinced that it's even broken?