Firstly, just because they potrayed a nicer future doesn't mean they weren't "adult."
At one extreme you had Picard refusing to sacrifice a single teenager to save the galaxy, on the other you had Sisko becoming a one-man mafia and CIA trying to add allies to the war. In TOS Kirk was led a nurse to her death because her living would have eventually meant Nazi triumph. None of those were easy decisions. There's many more - Measure of a man, even Tuvix, etc. Even outside the war context, The Inner Light portrayed the inevitable unspreventable death of a civilisation, by immersing the main character within that civilisation's dying days.
On the other hand, having characters say they're "making tough decisions" isn't the hallmark of "adult" or, more importantly, good writing. You can have all sorts of horrific decisions being shown on screen but they don't make any impact unless they are believable, you feel something for the victims or the ones making the decisions. Picard doesn't make any decisions in Inner Light, he doesn't kill anyone. But his pain at the end of that episode is more believable than anything the 2-D characters of Discovery showed when they betrayed each other for the tenth time.
Finally, the idea of the Federation - made explicit in TNG and then in the Bell riots episodes od DS9 - is about humanity being reduced to total depravity and warfare, and emerging from that with utopian ideals.
If I want gritty sci-fi, The Expanse does a much better job of it - realistic science fiction with realistic technology, realistic politics, better written characters, a very non-utopian world that you can easily see emerging from this one. I want something with the Star Trek name to hold on to the core principles that the series has held for half a century, it's no point writing that universe like all the others on TV. As
@Fingeredmouse said, Trek is about exploration and discovery - there was absolutely none of that in season 1.
