Discovery won't be painted into a corner. They'll go back and change time and stop Picard from destroying the Federation. Then Picard will die when Patrick quits the show, and Discovery continues in the 25th century while taking on the La Sirena gang, merging the shows.Well, if they go that route, they can spend the further Picard seasons to clean up that mess and everything would return to more or less status quo by the end. I don't think they'd realistically paint Discovery into a corner by depicting the fall of the Federation in a different series.
It might be better this way rather than writing Trek itself into a corner with whatever fate Discovery S3 shows.