I appreciate your perspective on a primary use cases of U++ .
However, I believe that incorporating simple animation and a rich set of styling abilities can still be beneficial to U++ users, even for those focused on engineering or business applications.

Enhanced user experience:
Simple animation and styling can improve user satisfaction and productivity, making software navigation easier and reducing learning curves.
They want to use your software because it's form and function are in-line.

Attract diverse developers:
A flexible styling system can appeal to a broader range of developers (front end and backend) , expanding the U++ community and encouraging innovation.
modern interfaces are exactly that "modern" and I feel it's important for a framework to offer that, after all it's fundamentally a UI "user interface"

Cheers