I think if sMerits will have an expiration date, people will be obliged or will become more active in giving merits to others instead of losing it, this will actually helps many members like me.
I would not like a hidden force to spend my sMerits. Unless I
am not a merit source1 I am not obliged by anything that I HAVE TO spend my sMerits in a certain time frame. However it's a moral responsibility. if I see a post is merit worthy and if I have available sMerits then I will gladly spend it.
1Becoming a merit source is kind of public responsibility to perform the job, IMO.