Problem is… if they added a “non-timed”, casual-friendly option for Mythic+, they would have to heavily nerf the rewards to offset the easier difficulty imo.
For example, a +14 currently drops 465 baseline gear, so if there was a “non-timed” casual option that option would have to drop lower item level gear at the end of the run… maybe 435-445 gear (just spitballing here)?
No way should you get top-tier rewards for “ez mode” non-timed Mythic+, it would destroy/undermine the incentive to push timed “competitive” keys.