Is it really that unfeasible that Ash could beat Cynthia? I don't think he will, for story purposes, but acting as if he doesn't have the ability to is being a bit presumptious.
Think about it this way. Ash is more than likely going to challenge the BF after the league. This could double up as a training arc for him challenging the Elite 4 and Cynthia.
So, when he inevitably beats Palmer (who I assume would be the strongest Brain-on par with Brandon) he could be ready.
Also, the beginning Movie 12 showed us Palmer battling Cynthia for a moment, seemingly an even match up from what we saw. So what's to say Ash couldn't beat Cynthia if he beat Palmer and has beaten Brandon? It seems to me that
top Frontier Brain= Champion more or less, in terms of strength. Not to mention Ash would be using reserves most probably.
Going by my thoughts on it, Ash will have the capabilities to beat the Elite 4 and potentially Cynthia (although as I've said I don't think he will for plot purposes).
Long winded post over