Rotom's viability goes beyond countering Talonflame and Pinsir. Don't talk like that's the only reason to use Rotom. He can most certainly still run offensive sets, I use them and I run into them often. I know personal experience is a bad indicator of viability, but still. Even if your statement that Rotom has to run PhysDef was true, I don't see how that effects his sets that much. TrickScarf, ChestoRest, and Pain Split sets don't care much about the EV spread and Specs sets can just run some defence if you really need it. And let's not even get into the various status moves he can use, Thunder Wave and Toxic are still things to look out for.
I also have no idea what you mean by Rotom being a pokemon that loses health every time it enters the battlefield, since every pokemon bar Magic Guarders lose health when under the same situation you stated, and as such that argument cannot be called relevant.
The gen shift was most definitely good for Rotom, why the hell do you think it's at the top of the December usage list? New threats to check, a slower, more physically inclined metagame, the loss of rain hurting other bulky waters much more seeing as they got literally nothing has all helped out the washing machine.