Michael, I understand that commercial reality. I face it too, and my team and I often brainstorm different ways to explain the full value of what we’re offering to clients, when much of that work is unseen. (I would love to hear how other shops are successfully communicating the value of this kind of work!) Part of the point I’m trying to make is that the burden of that is on us, not on the client. Responsive design, accessibility, and performance should be core principles, meaning they’re the non-negotiable underpinnings that ensure that our final products are serving both the clients and the visitors, even though they’re unseen. To leave them out is akin to building a home without a foundation.
When we aren’t building according to these principles, we’re giving our clients a sub-standard product, and everyone suffers. The visitor suffers a frustrating experience and leaves the site, in search of other solutions. The client suffers this loss but doesn’t even know that it’s happening. As a result, our whole industry suffers as the affect snowballs.
As you acknowledged, we all know a client isn’t going to come back later after the project is done, and ask you to remediate their website for accessibility - unless they get sued, in which case they’ll be paying for a lot more than just that remediation. It is much faster and easier to build a site with accessible markup and principles in the first place, as a default practice. I fully believe that coding that way doesn’t have to be more expensive in the long run.