No, sMerits do not expire, although there is a reference to this that opens a door:
Nevertheless, that door has been kept shut from the beginning, and I’d be surprised if it were to actually occur at some point in the future.

What is likely normally monitored is the monthly flow of sMerits in the system amongst other dimensions (i.e. somewhere along the line of some of the things show for example here, but with inner tools/queries). So long as things are reasonable on a monthly basis, there is likely not much of an issue with/of people hording, as Merit Sources do quite a lot of pumping sMerits into the system. Forum members will likely make threads on Meta to alert when things are not going well in terms of sMerit flow, and that has, in the past, led to changes in Merit Source numbers and quotas to pump things up a bit in the past.

Merits are kind of an equivalent to Karma and Likes, but with more semantics and scope as I see it. It aims at pointing at posts that are interesting for some reason or other to the meriter, and it shouldn’t really have any more transcendence than that.