This niche is pretty well served by some of the portable gaming PC companies that have added business netbooks to their lines. GDP has the mini 8" transformer which isn't bad. The 4 just came out with ridiculous specs for such a small machine, but with poor battery to match. The 3 can be had cheaper with less powerful CPUs and better battery. I'm using a onexpro netbook 5, a really high quality 10" that compiles large rust code bases fast enough for me to be happy and also doesn't suck at all to type on, battery is fine at 5 hours with really fast charging.

Tablets with portable keyboards replaced that niche, mostly.

This is served by tablets with a keyboard, like the iPad Pro or the Samsung Galaxy Tab.

The PC market is too enshittified for this to happen. Companies no longer differentiate on hardware but on "subscriptions and services". Portable gaming machines like Steam Deck, ASUS ROG Ally and some indie OEMs are the closest thing we have to a mainstream mini PC.