Buying merit will be a temporary thing, eventually all farmed accounts will run out of sMerit to sell, and won't receive new merit unless they create quality posts. Some users could still rank up with bought merit, but the majority won't be able to do that. And as bitperson already pointed out: that's how it is intended.
I think it will take a lot of time before that happens. Note that we have a lot of unused accounts that are being taken over by hackers. Those are mostly people who had accounts here before the database leak. Now these accounts can be used to spam merit till the end of days. I'm pretty sure it's already being done.