If the 6-8 fight rule is so critical they really should have a red letter warning in every book published saying "classes are balanced on 6-8 fights per long rest, and short rests every 2 fights. Find a way to make this happen or your games, especially at high level, may be reduced to a steaming pile of doo doo".
Keep in mind it's 6-8 encounters or equivalent. With the 6-8 based on "hard" encounters. If you overclock your CRs and push much harder threats you can do less. But you have watch it. So called "deadly" encounters may easily not be - as in they can be cakewalks with a bit of proper spell use and luck. Right up until you overclock the CRs just a bit too much and then you go from winnable to likely TPK, FAST.
That's one huge difference between martials and casters. You KNOW what martials will be brining to any given encounter. With casters, it depends on what happened earlier in the adventure, what resources (spells) they are willing to expend and how well they time them (as well as a bit of luck re: saves etc.) - the difference (especially as levels increase) can be as dramatic as a cakewalk encounter vs. a TPK/near TPK encounter.