I know you guys want what would be best to the game mode but you have to remember that first and foremost Riot is a company and as a company the goal is to generate money.
Letting people play the new champion for free will, without doubts, hurt the sales. Back when League was still smallish you could play the new champion for free the first week after release, the last one was Gragas which came out at the same time as pantheon, Gragas was free, Panth wasn't and around 80% of the people bough Panth while only around 20% bought gragas, since then every champ that has come has been bough if you want him to play him out of rotation, do you know why? because people buy them on a whim, they like the concept, the lore, the spotlight or whatever reason you might think of but they won't necessary like the kit and playstyle. If you get to play him and you don't like him then you don't bough it, simple as that, so it will definitely hit the sales.
Secondly, you think people spaming ARAM to get to play the new champ is a good thing but in reality it will be awful. The players will queue on ARAM, hope to get the new champ, if they don't they will reroll twice and if still doesn't happen to be the new champion, they will quit the queue and start a new one in 5min, once the queue is expanded to 30min wait for leaving (3rd quit) they will just play a single game (with they might or might not troll, but chances are they will troll most of the times) do you have any idea of how frustrating would it be to play aram during release week?
Third, there are a lot of players that only play ARAM, and I don't mean ARAM smurfs I mean players that has Quit SR because of the amount of toxicity there is on there and play only or mostly ARAM which is a more chill mode. If you unlock all the champions for free then what initiative they have to buy new champions? For eventual ARAM players like myself it will probably be the best but you have to remember the amount of money loses those changes will cause.
