Assuming all teams that are 1-3 are going to lose:
Should Vegas blow it up? Caps? Jets?
These are all considered good teams, and reaching the top 8 is a fine achievement. One could even argue that for this Leafs team, getting to the conference finals would have been overachieving.
They have the cards they got dealt, you can't really force it. Letting Marner walk would likely be a poor option. Whatever they sign for UFA in place of him would likely not come even close. Star players in general tend to be good value for their contracts so they should be kept. Also, they don't have first round picks for the next few years, so rebuilding isn't an option, either.
The better thing to do would be to just change the supporting cast. Not every team is destined to win cups right away, and you cannot force things shortsightedly. If they end up having to rebuild, it should be done in 3-4 years when they actually have picks, not now.