I am not a fan of a 5mwd, but I would walk right then and there.
Yeah the idea the DM is breaking the trust as much as the players allegedly "abusing" the 5MWD there.
The DM's response should be "Didn't we agree not to abuse the 5MWD?" and then actual conversation about it, because I don't for even a FRACTION OF SECOND believe the players would genuinely go directly to abusing the 5MWD after agreeing not to. That's not how humans act in a social situation. The reality would be that the session 0 had failed to get the players and the DM on the same pages as to what constituted "abuse". Also the DM clearly isn't as familiar with 5E as he thinks, because 6-8 encounters is not actually how 5E is intended to be balanced. This was literally explained at some length in a video with I believe Jeremy Crawford in like, 2018. I might even be able to dig it up. He says 5E is intended to actually function well with 1, 3, 5 encounters, it's not married to 6-8 "normal" encounters. Now, I think if Crawford is being honest and we should assume he is, then he messed up badly by including both Short Rest and Long Rest characters (pick a lane buddy!), and making short rest an hour, but that's a separate issue.
In general I think the suggestions that this be solved with play style stuff or "agreements" are kind of missing the point of the thread. This is not something impossible to solve mechanically. It's not something that's an issue in every game.
I really do think step 1 to resolving this issue is probably to get all/most classes on the same page re: long/short rest. By far the easiest way to do that, is simply to multiply Short Rest-refilling resources by x3 (i.e. +200% for two imagined Short Rests) and make them Long Rest. It's also nice because you can kind of apply it "ad hoc" as it's a very simple rule of thumb.
Once you've done that, it's a lot easier to look at what's gained from a Long Rest, and how a Long Rest should and should not work. It does temporarily make the "nova" issue worse, but I dunno, I think any attempt to retain Short and Long classes whilst fixing the resting issues I definitely agree 5E has is somewhat doomed.
(The alternative is to remove Short Rests the opposite way, by making SR resources just renew out of combat but there be less of them, but I think that's a lot trickier to design right.)