Serious question: who is paying me as a freelancer or small business owner to work on mobile accessibility? It's not the client.

The amount of work and hours put into developing and maintaining (nevermind learning from scratch) is hugely exceeded by the 10-15% potential increase in revenue, which I'm dubious about to begin with. Is the 200 hours/year it would take to maintain at $100/hr worth $20,000 in incremental revenue? I just don't seethat happening.

I don't disagree with most of the article, but the expectation that "you should do this without considering opportunity cost" makes the entire argument moot.