You're confusing the diversity of champs available at a given point in time with the diversity of the experiences you have playing the mode over time.
Look at the Blood Moon mode or the Project Mode. They were new experiences that you couldn't have on Summoner's Rift in part because you had access to less champs. To some extent, the same will be true in ARAM, the experience you will have in week 5 will be different from the experience you had in week 2 because the pool of available champs is different.
And for every champ that is available, it is indeed more troublesome. But the gain from any additional champ in the pool sharply decrease as the numbers go up. Quick example: ARAM-only account has 10 champs unlocked. With 10 champs in Free rotation, so 20 total champs, he has 1 in 2 chance to get a champ they want, so 50%. By adding 20 more champ to the free rotation, you go up to 40 total champ, so 1 in 4 chance, 25%, so you "gained" 25 point compared to the previous value of 50%. Now if you had 20 new champs again, up to 60 total, you have a 1 in 6 chance, so ~17%. By adding 20 champs again, you only gained 8 points instead of 25.
Going from 14 to 42 champs add a lot of value. Going from 42 to 140 would not add much more.