What’s strange is that by your definition, any other end-game pillar of content that provides a comparable item-level track to Mythic Raid makes mythic raiding “unrewarding.” There’s no solution to that problem if that’s how you see it because the dev team looks to be committed to providing multiple end-game pillars to suit PvPers, Raiders, M+ers, and now Delvers. The only solution to your issue is that only Raid would provide myth-track gear and that’s not a viable solution for the devs to consider.
I do think there’s an issue with +8 crests being infinitely farmable (up to the seasonal cap) and if you only do Raid then you’re unlikely to hit that seasonal cap. You should be able to hit the season cap at some point if you kill 6 Mythic bosses per week and never miss a week.
M+ probably needs some form of lockout based on track-level like: limit of 8 myth-track opportunities (not actual acquisitions) per week from end-of run, somewhere in the +12 to +14 keystone level. That means adding myth-track to end-of-run at some keystone level since M+ only gets one piece per week from the Vault (which mythic raiders also get on top of chances at myth-track gear on each boss killed).
More opportunities to farm crests in M+. More opportunities for myth-track gear in Mythic Raid… they could balance those two issues better and Mythic Raiders would still probably complain (seemingly such a bitter sub-segment of the PvE community; nothing’s ever good enough).
Edit: Looks like crests will be a bit easier, but still no buff to crest drop rate in Raids 