We are losing out on so much.

We are yes... Turbodiesel seems to be the answer even though a rebuild is not possible so they need a replacement very often :( On the other hand the unleaded gas engines like Rotax seem to be super reliable in the ultralight category, they are just not certified for real GA. I would imagine that slightly scaled up they could be a real workhorse.

Or perhaps a really small turboprop. I've seen really tiny jets so why not stick a prop on it (because jets are too inefficient at low altitudes). Might help with noise too.

There's so much innovation we could be taking advantage of :'( But we're still using engines designed in the 50s with lead (replacement) and manual choke/mixture that cars have deprecated 30 years ago.