Opportunity cost is a huge problem but it seems like an even more basic issue is just the suitability for the underlying problem. The hype around blockchains lead a lot of people to suggest them as alternatives for relational databases or digital signatures, and even if a high-quality, tested, easy to use, well-secured system magically appeared it'd still be an inferior choice for any problem which can be solved with those tools: transactions take orders of magnitude longer to clear and the capacity limit is orders of magnitude smaller, customer service and fraud are hard problems rather than routine operations, etc.
Even if the opportunity cost was zero, the choice wouldn't make sense unless you had a problem which cannot be solved well with mature tools.
They're definitely related problems — I just find it fascinating how often enthusiasm leads people to completely downplay this category of problem, often while spending non-trivial amounts of energy trying to deal with them
