Then that’s their skill ceiling, for the time being. They can keep running those key levels while gearing up and skilling up. Everyone’s progression stops eventually. It’s not like if you can’t time a +2 then you are banned from M+. No, you get another M+2 key.
Flightstones are basically free. You can get them from almost anything. You don’t need crests to upgrade them.
Except it isn’t actually a M+10. Even in Blizzard original post they stated it is similar to a M+8-10 except it doesn’t have a timer or affixes. WoW head removed the “8” and just kept the 10. If you take that and remove the affixes and timer, it’s nothing like the previous difficulty of an 8 or 10. You need to understand how big of an impact the affixes had on players who were skill capped at the 8-10 range. Now also remove the timer. The M0 being “liken to M+8-10 minus the affixes and timer” is not as big of a gap as you think it is.