Is the "All Riders" name by itself really enough to dilute the idea? According to the reports about the movie, the "All Riders" in Let's Go Kamen Riders apparently didn't get much focus. It was mostly just Kamen Rider (1 and 2), Den-O (+New) and OOO.
Isn't that doing more to dilute the brand and idea than a new tv show featuring team ups with past Riders would do? If someone went to watch that movie for one of the Riders besides the ones from those three groups, he'd probably be disappointed and less inclined to follow another "All Riders" title again. On the other hand, people who don't know those Riders probably won't be interested in them just due to brief silent participations.
The merchandising line seems to be doing great with their current "All Riders all the time!" approach, like Rider medals, switches and tons of crossover games. It's not like I'd want to see that going into the tv show, but I think if "All Riders" actually received focus in a production for once, it wouldn't be a "tiring" concept.
Now, after they actually do a big tv show, then giving a rest to the idea would probably be appropriate for a while, but movies don't seem to able to take that concept far enough by themselves.