I agree with you, however Riot has kinda stated that they're not increasing the amount of free champions on the weekly rotation.
At first the proportion of #free-to-#ofchamps was 10/40 which means that 25% of the roster was free, now it's down to 10/122 which means that roughly 8.2% of the champions are free to play/test (the number of champions represented by the free rotation was about 3 times larger those days.).
I think that rotations should serve as testing champions for old players and learning champions for the new ones. You can't master a champion in a weeks worth of time so I believe the entire concept of rotations is kind of out-dated since you would actually be trying out the champion.
What I would like would be if we could somehow get meaningful champion price reductions, have all champs free in customs so that we can test them, or have a number of over-time-renewable "trial tokens" per champion. I feel that there are many many more solutions than just these three, and if Riot wanted to, they could pull it off.
I am speculating that part of which they are keeping the rotation small is because of IP grinders. Under this scope, if Riot were to increase the number of free champs playtime would potentially decrease -> revenue would decrease in response. Another part of this fact is because if content is easier to get you get less incentive to spend RP, and a substantial amount of the playerbase purchases RP quite often.
Gonna tag this thread with @Hippalus / @Wizardcrab / @RiotHippalus / @Riotwizardcrab/ @Riot in case they ever decide to show us they care about free content by engaging in meaningful discussions with us.