I wouldn't say it's correct to say that GEOS isn't particularly sophisticated. A lot of (certainly not all) GEOS algorithms are now ported from JTS, the primary author of which is Martin Davis (aka Dr JTS), who works at Crunchy Data, who provide the PostGIS extension. So the chain (again, mostly) goes JTS -> GEOS -> {PostGIS, Shapely} -> … . Martin's work is at the cutting edge of open-source GIS-focused computational geometry, and has been for a long time (of course, industry has its own tools, but that's not what we're talking about).

I can sort of see your point about the merits of global, spheroidal geometry, certainly from a user's perspective. But there's no getting around the fact that the geometry calculations are both slower (I'm tempted to say "inherently"…) and far more complex to implement (just look at how painful it is to write a performant, accurate r- or r*-tree for spherical coordinates) along every dimension. That's not going to change any time soon, so the projection workflow probably isn't going anywhere.