I'm pretty sure they've indicated that they're looking into champion mastery for alternative game modes, including ARAM, so it's probably not that you'll never earn them, it's just lower priority than getting it all up and running for SR. Hopefully it'll be all fixed up eventually though.
As for ranked ARAM, that has issues with ARAM accounts that only own the ARAM OPs, plus it's not necessarily fair even in theory. SR ranked it's two teams consisting of similar MMR people, who take turns picking their team comps and so can in principle have counter picks and strategies to deal with what comes up. With ARAM, although it's similar MMR people, theres no control over the champs so people could get stuck with their worst champs and/or end up with a comp totally countered by the opposing comp, and theres not much they can do about it. ARAM mostly being constant teamfights as well, theres limited strategy (compared to rift), so theres minimal room for strategic outplay (contrast with rift where if you're behind and/or your teamfight is weak, you can just waveclear and split and collect dragons/baron and win through those instead of teamfights).