hi Christopher,
I think it makes more sense to re-use web frameworks instead of SAP even trying to develop one. About UI5 and feedback, I don't think SAP wants feedback on their library - it's closed source in that aspect, meaning outside contributors have no say where it's headed - they accept "bug fixing" but the process seems rather "painful" and you can read the FAQ and the gh-page related to "Contributing" to OpenUI5 to make up your own mind.
Cheers,
Dan.