I agree with @mbauman, @rafael.guerra and @Juan that there are often legitimate reasons for adding posts to old threads. Absolute solutions, like automatic locking or unsubscribing participants, fail to serve these valid use cases, for little benefit (eliminating a very small amount of thread ressurrection that currently happens).
I think the “Revive this topic?” notice is the right way to address this problem, and for all we know (and countering @tbeason’s suggestion), the fact that we do have a small amount of thread revival may well be evidence that the nudge works well — though I concede, it isn’t 100.00% perfect. 
If anything, I’d suggest tweaking that notice to add that, in addition to notifying a lot of people, reviving an old thread that has dozens of posts is more problematic (as it may require a lot of context-gathering) than an old one that has only a few. But that’s just my 2¢. 