I don't put much stock in the "evergreen" label. D&D Essentials was advertised as evergreen too. When is the next print run of that.Mearls has used the word evergreen on multiple occasions, actually. Doesn't mean there will never be a new edition, but they don't use 5E in any marketing for a reason. If there is a 6E, they want it to be backwards compatible and painless for existing players, that is, backwards compatible. Newer editions are probably as bad a bellweather as the older ones.
For a new edition, I would forsee the main change being in the specific exceptions to the general rules, such as Classes and Spells, but no essential change to the superstructure of the game. So we might get a new Ranger built from the ground up, but not new rules on stealth or something. And a 5E Ranger would be able to come in and play next to a 6E Ranger. A tenth anniversary set with new art and refurbished options that is a sixth edition seems possible.
A 5.5 is probably not going to happen. Another book of options like XGtE, or two, might happen, but no overhaul. Mearls has said as recently as last Tuesday that the PHB has sold so well, and is continuing to sell so well, that they will not do anything to remotely appear to invalidate any of those purchases.
Nothing is forever except editions that are no longer actively supported.