I really doubt they would lock max level to 60.
While clearly what the number attached to the max level means nothing anymore. Their is still a imaginary sense of progression that would be lost if each pre patch coming forward take us back to 50.
It took them 16 years for a level squish, I think we a have some time before another one is needed. And most importantly, they can just tweak the experience gain for the leveling to always be at the same speed with chromie time (so that when next prepatch comes, you can level 1->60 doing just one Chromie zone at the same speed as current 1->50)
And since they will most likely never add any new talents rows, there is also no need for a 10 level jump. Going forward I think a 5 level jump is what would be best. Just for the feeling of progression and it even integrates well with how leveling is now (3-5 zones in expansions "continent", so a bit more than 1 level per zone sounds good to me)
With 5 level gaps it will take ~8 years to reach max level 80 (accounting for ~2 years expansions). At that point, we could squish back to 50 if wow's still alive.