You haven't said any really good reasons, and I have, I think. Which you haven't addressed.
I'll give you examples: Paris/Bercy finishes early November. That's the end of the year. YEC can be any time before 1 Jan of the next. Could be a complete one-off and rotating between continents and hemispheres. Why not? And, thereby, it could change between outdoor/indoor and surfaces. What would be wrong with that? Honest to god, you whinged endlessly just about the slowing of the grass this year. You'd be out of your head if the last tournament of the year were always played on clay. Why would it be so bad if it rotated, not just surfaces, but locations? It kind of is the floater between the Majors. Why should there be yet another one on HC? And yes, why couldn't it be alternatingly on grass?