Innovation.
D&D is the industry standard and pretty much drives innovation. Each edition of D&D inspired many RPGs. Like the "similar games" you mentioned.
Even if 5E would be perfect (which it clearly is not) I would want a new edition to have innovation. The RPG scene is already lacking in innovation and gamedesign compared to other types of games (boardgames, computer games).
If D&D also just stops innovating, the gamedesign of RPGs will fall even farther behind.
Also in top of that, 5E has many fundamental flaws/inellegances, it got better with 5.5, but some things cant be fixed without a start from scratch.
- Having normal starting level be level 3, because the first 2 ones are just tutorial /hemmed because of multiclassing
- Not too elegant monster design and encounter building, again it got better, but still far away from the elegance of 4Es
- Martial caster disparancy. Not only power, but complexity. Martials are simple, casters complex.
- Combat too simplified, since non stacking of combat advantage, leading to many inelegant "add dice to roll" effects instead of a more elegant general system.
- Having unequal power progression (trippling power from 1 to 3, doubling from 3 to 5, doubling from 5 to 11...)
Of course 6e will also has flaws, but it may have different ones, and will again inspire many new games.