There's a big difference between "sub-optimal" and "objectively awful."
You can tolerate and even enjoy some of the former but the latter is unplayable. Warhammer 5th Ed. in its final, ultra-cheesy, beard-wearing incarnation fell into the second category. I reached the point where I was turning down gaming invites because laundry was more enjoyable.
Similarly, 3/4 ed. 40k achieved a level of counter-intuitive tactics that made me loathe it.
If 9th ed. 40k reaches the same plateau that 6th edition Fantasy did - flawed, but workable - I can see people getting back into it, particularly if they've got usable armies on hand.