With an exception of web there are very solid GUI cross-platform (linux, windows, mac) frameworks out there. Qt is even pretty good at targeting native look.

> All of this translates to two pretty much essential features for any startup: speedy entry to market (MVP) and low cost.

Yes, web technologies let you build MVPs faster, that's literally in my original comment. However, in order to provide quick MVPs these technologies inherently sacrifice long term maintainability.