Dungeons outside of the M+ system serve little purpose in the modern game. You might run them once to complete a quest or get a feel for the dungeon mechanics - but nobody is actually running them as their endgame.
Casual players can get much better loot with less headache from the delve (and soon Prey) systems.
Less casual players won’t do M0, or even M+2 to M+5 for very long, their endgame will quickly become higher M+ and farmable Hero gear + Myth track items in the vault.
Consider making M0 equivalent to T11 delves - wherever that ends up being. Likely Champion drops, plus a Hero item in the vault, and Hero crests. Add M0 to the dungeon finder while you’re at it. That would give casual players more variety in their endgame, and give them a reason to run dungeons.
People who’s endgame is delves and Prey aren’t likely to start meaningfully engaging with the M+ system. But they would endgame with moderately challenging non-timed, non-keystone dungeons - especially if they were available in the Dungeon Finder. The loot just has to match what they can get from delves and Prey.