"D&D editions last about a decade, so we're just a couple years from 6th Edition."

"3.5 and Essentials came after 3 years, so 5.5 is due any time."

This is cargo cult thinking. Edition changes were business decisions that had nothing to do with a ticking clock.

In virtually all cases, the new edition trigger is pulled due to declining sales, which have typically been due to TSR/WotC saturating the supplement treadmill and needing to reboot.

(The one exception is probably the original 1974 boxed set -> Basic/AD&D products in '77. Sales were still the motivator there, but it was more about creating a product for the more mainstream audience D&D was tapping into.)

5th Edition's release strategy was specifically designed to prevent the saturation of the supplement treadmill and extend the life cycle of the edition.

It seems to be working.

(3.0's release strategy was also designed to do this, but leadership changes at WotC saw a shift in direction which resulted in a 3.5 that was specifically designed to MAXIMIZE the supplement treadmill to increase short-term sales.)

Another factor can be a perception that D&D's production values are out of sync with the market, damaging their market dominance. (This was true for both 2E and 3.5.)

This is clearly not the case with 5E.

In any case, all reports indicate that 5E is still experiencing huge revenue growth every single year.

There will be no new edition as long as that remains true.

You don't slit the throat of the goose that's laying golden eggs.