"MVC(D)" and "MVVM" describe how to multiply the amount of time it'll take to develop an application in them. "MVC(D)" will take at least 5x the cost of a Domino app. "MVVM" will take 6x (because it also has an implicit "D" in there). Never mind the risk of completely new development, that's really how much it costs.
The reasoning is pretty easy to see: Domino development, even in XPages, tries to unify the development interface. But if you're working through view, model, controller, and database, you're essentially making four different development efforts consistent with one another, often using at least 3 significantly different languages, and two more with different perspectives on coding.
(And that's actually a rather low, back-of-envelope guess. I've seen 20x & 100x in the past 5 years. I believe that's due to inexperienced developers, but I'm not sure.)
It's truly up to them. If your customer wants to spend more in development to migrate their capabilities, that's great! Just make sure they're on board, because the development cost will go up.
To me that's the main attraction of Domino. It is so fast at development.
So to change tech, you end up with a lot of work to do. Yay novelty!
Not that the change is futile: it isn't. Domino is aging, and IBM's commitment is weaker than I've seen it. There're other advantages, too. You can hire programmers who are much more familiar with the latest platform, tech, package, dev environment, technique, etc. But those advantages will not alleviate the higher cost & time to develop an application. Removing this technology has a considerable cost. Its advantage is real; Microsoft and Java sales pitches that they'll do the same ... are not so real.
So, you know. Make them aware too, so that when the novelty wears off, you can prove you estimated it accurately.