I like the subspace theory.
Could it be that hundreds or thousands of years of warp travel took its toll on the environment and subspace was just wrecked making warp travel impossible?
It makes sense that warp travel would have side effects.
It was explained that regular Warp drive damages subspace, however, it is highly unlikely that this would be the reason behind 'the burn' because Voyager for one thing has variable warp nacelles which circumvent that issue, and it stands to reason that Warp engines on other ships (especially new ones) would be modified and designed in a way to prevent this kind of damage from occurring in the first place (in the meantime, the Federation and the Klingons did agree to put restrictions in place so that only Warp 5 would be used, whereas higher warp speeds would be used in case of emergencies or if the situation deemed it necessary).
Besides, it was established in TNG that the particular (narrow) section of space was afflicted due to it being the only viable option to travel safely in that region.
It is highly unlikely that this would become a problem throughout Federation space because first off, space is HUGE, and second, most ships travel separately and are usually scattered throughout the Federation... most ships don't travel en mass at high warp frequently (only during war time would this happen likely, and I'd be surprised if Starfleet/Federation couldn't find a solution to REPAIR the subspace damage).
Also, why would only the Federation be affected? This would likely have to happen throughout the Galaxy. The Dominion certainly doesn't strike me as caring too much about the environment, and its been in existence for 2000 years.
You'd think they would have been cutoff from the rest of the Galaxy completely and isolated in their space.
Maybe beings like the Q were playing literal snooker with suns and planets. Much chaos happens later they can't even fix.
Like what?
Apart from the civil war in the Q continuum, it is unlikely that Q would be unable to fix large scale problems or certainly something on a galactic scale).