A few advantages: self-documented classnames, zero dependencies, classnames are reusable across projects, removes the need for a CSS preprocessor or bundler, no need to switch between HTML and CSS file while developing, etc...
Your technical stack should not be frozen. During years, it will probably evolve, either to use some promising features of new technologies, or to still be able to hire. For instance, hiring some Cobol developers is really hard (and then expensive) these days.
More important than technologies is testing. If your whole app is well tested (with both unit and E2E tests), you won't be reluctant to upgrade some part of your system because it doesn't answer your needs. Moreover, it would bring a lot of reliability and would be self-documented. That's the biggest point IMO.
For nowadays mood, we are really glad using React, Node, GraphQL and PostGreSQL for all our apps.