This is not a true dichtomy (quintomy?), but just a point for discussion.
Keying off of this market here, many voters selected, "Decade Plus Long Markets Are Unimportant," -- even if you didn't select that as your favorite, you might choose one of the reasons given in this poll as a hypothetical. You can go back and change your previous vote if you already voted here of course too.
Other question for the converse answer if you wish to answer as well:
100% opportunity cost for me. I think the long term markets are interesting and valuable. It's often actually easier to make long term predictions than short term ones (e.g. will the s&p 500 be higher or lower than today in 1 week or 10 years).
But if I'm short on funds I'll tend to sell my long term positions and move them into short term ones. Long term markets don't move very quickly so it's also easy to think (well I can just sell x for now to buy y and once y closes I'll buy x back at roughly the same price). I might never actually go back and buy x though.
@JeremiahKellick I also agree with you. I also think that long term markets resolve pretty rarely which somewhat ruins the purpose of a market (which is to have market resolutions as a form of continual feedback).