Can we agree that Crit matter less during combustion (it does convert to mastery, hence less and not none)?
If you look at a fire mage's damage profile, you'd see peaks during combutions, and low valleys outside of out it. Almost all of our damage is focused into that combustion window, and there, crit really doesn't matter as much as the other stats.
You could have over 50-60% crit, but this would only really be useful outside combustion and execute, and it's still not going to matter.