I would just like to point out that in material UI is an entire visual approach and system, and the defines a lot of consistent ways in which different widgets would look and behave. If we were to use this, we should use it for the entire application, and that would be a relatively big change.
Bootstrap is also a complete visual system, but it basically looks just like everything else, so Shifting the reference application to be based on bootstrap shifting the reference application to be based on boots drop can be done without much visual change.
The reason that Stephen wants us to adopt bootstrap is so that we’d be using a standard technology that devs who are new to OpenMRS already know. (Personally, I don’t think it’s worth the investment to make that shift, because the current refapp styling is adequate as far as I’m concerned. And bootstrap + angular or react still requires a third-party Library beyond the bootstrap one. But it’s not my call.)