Anyone can sit back 1000 range and spam poke as Lux or Xerath.
I don´t think you understand how the mastery system works. You only had to read the whole thread, OP actually responded to this argument.
It will be like summoner´s rift, the system will compare you to other players. So if those champs have high cs,kills,damage,etc. then well they will actually be the hardest to get a mastery token with, because they get compared to the high performances of other players and you need to achieve similar performance then which means it´s way less forgiving then a champ which actually struggles on the map.
Reason being S ranks in ARAM don't really prove any mastery with a champion.
Yeah and I got mastery 6 on most supports by playing them at top when I was originally queued as a jungler. I did this originally to see if it might be some new meta, it was at a time where frost queen would be build on ranged champs who would go top and while I was playing 2-1-2 strategy I just decided for an easy token 6 collecting (I´m collecting mastery 7 for every champ). And you can do this with other champs going off-meta too, just where players should be normally bad with those champs.
So I don´t see a reason from this point of view to not enable them being earnable in aram.