Given that classic basically doesn't have any development going on, the subscription alone is more than enough. People playing on Classic are literally just paying for vanilla to be playable on modern machines and to play on the server. That $15/mo covers less than 10% of the cost of the servers IIRC. It's pure profit, and people aren't even expecting updates. It's a cash cow and Blizzard would be extraordinarily foolish to try and get greedy with it.
That said, this is Activision we are talking about, so be wary...
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Ultimately he had a strong point. Businesses are never happy with just being profitable. They want profits to go up, not continue to be successful at the same rate they are. That stock value has to rise, it's not enough for it to merely retain its value and offer decent dividends. Greed has killed off too many MMORPGs to count. It's not unfathomable it could happen to WoW Classic, and frankly, the people who would let it happen do not care if it hurt (or even killed) Classic if it benefit them in the short term.
This isn't the Blizzard of old we're talking about. It's Activision. And activision is both greedy and foolish with how it handles its franchises. A sane person knows it would be a bad idea for the game, but many businesses only see things in $'s, and only in quarterly statements. They'll do anything to push the margin and hope they can fix shit later. They don't have a rat's ass whether WoW classic died. This is especially true if they felt they could squeeze more money out of it by doing something underhanded than gaining money over its natural lifespan... or even get a similar amount of money immediately instead of waiting that long.
And I guarantee you, that as little as Activision cares about any of its franchises, the investors will care even less. It's all about the money. In short, don't underestimate corporate greed.