If you think wows decline all comes down to two individuals at Blizzard, I would call that a bit shocking to say the least. Theres a pletoria of reasons why wow has declined over the years, just as it wasnt one reason alone to why wow reached 12 million subs during wotlk.
If you draw the line at cata, one could argue its a cultural shift around that time in how MMORPGs are played & consumed by players overall. Maybe the old school MMORPG player started to fall behind at this stage? Maybe the game has changed just as gaming overall has changed?
We can talk about wows decline, its failures, how much it stinks and all that - but it doesnt change the fact its the only big mmorpg still going fairly strong in a fiercly competetive gaming landscape. The fight for a gamers attention has never been harder than now.