I think that MOBA's definitely had an effect, but not a drastic one. They're two completely different types of games and they don't scratch the same itch. I know people that quit WoW, and LoL became their new addiction, but they didn't leave WoW for LoL, they just quit WoW to save money and picked up LoL to have a free game to play and be competitive.
I feel like the only p2p MMO that still really has any chance is WoW. Their numbers will skyrocket when Legion hits, and then slowly dwindle off some with the people that realize they just don't like WoW any more. They'll maintain subs if it's a good expansion, and they'll lose more subs if it's a crappy one.
I feel like the pay-to-win, f2p, micro-transaction, and in-game purchase MMO's are on life support, but not WoW quite yet. I think WoW still has a lot of life in it. WoW has an infection, and Legion is the antibiotic. If it works, WoW will be all better. If not, it might have to amputate (get rid of the monthly fees) to maintain subscribers.