So despite Season 3 looking horribly un-Trek and just generic grimdark Sci-Fi from what we've seen so far I have thought of the way the premise could possibly work and actually be good.
So we know that the Discovery has gone to some awful future where the Federation has somehow collapsed in on itself in barely a century, but what if this wasn't because of an attack or some kind of political upheaval (neither of which should realistically be able to bring down the 31st Century Federation in such a short timespan given its size and technology by that point) but was in fact due to surgical attacks throughout the timeline on the Federation's history, with each one rippling through time and making the future more and more bleak. We know that even in the 31st Century the Federation seems vulnerable to historical changes due to 31st Century Earth becoming a destroyed, lifeless wasteland due to Archer being removed from history.
At some point a few planets realise what is going on and are able to develop technology to defend themselves from additional changes in history, but by that point there's barely any of them left and they lack the resources to both defend themselves from the forces in the present attacking them and investigate the changes in history. So Discovery comes along and presents them with a unique opportunity. The ship is basically useless for helping to fight off 31st/32nd Century attacks in the present, but with some outfitting of whatever future tech is compatible the Discovery can be sent on missions to various points all throughout history to correct the timeline, leading to the future being gradually restored. At the same time, Discovery is tasked with unravelling the mystery of just who attacked the history of the Federation and why.
Eventually this could be revealed to be any number of things. Maybe one of the ancient fallen empires (Like the Iconians or the T'Kon) have returned to reclaim their territory but needed to weaken the Federation first. Or maybe it's a totally new threat from outside of the Milky Way. They came with the intention of conquering/enslaving the galaxy but found the Federation there, who at that point would have spread across most of the galaxy and have incredibly advanced technology, plus by that point likely millions of staships, all of them with the capaibility of jumping to any kind of invasion point within minutes. So with that many ships and an empire with the resources of an entire galaxy at their disposal any kind of direct attack is doomed to failure. So they decided to weaken the Federation throughout history, whilst leaving at least some of the technology, and most of the people, intact for conquest.