Having a lone Rider would also leave a gap in the toy line that would have to be filled somehow - and trying to fill it with something new would be a risky move, since there hasn't been a single-Rider show in fifteen years. I know a lot of people don't think the toy sales are/should be important but, face facts, they are. That's where the money comes from to continue the franchise.
Amazons looks likely to end with Haruka as the only surviving Rider so you may well get your wish for a lone-Rider show next year anyway.