The young viewers would be pissed off because a straight cis man is playing a trans.
I'm astonished at how few people understand the danger of this type of thinking. "Only trans can play trans" "Only gay can play gay" sounds great, until someone counters with "Trans can only play trans" and "Gay can only play gay". Then the industry comes up with several years worth of no gay or trans characters, and those actors find themselves with nothing to do. While their inevitable complaint is actually viable, they can't really prove discrimination because it could be argued that if they want gay and trans characters for them to play, they'll just have to write them themselves, as the writers already getting work in the industry are too busy writing straight, cis characters to bother with making characters gay or trans just so those actors can get work. It's a very slippery slope.