If by “seamless” you mean that wireless data speeds can soon match locally attached storage, there will still remain a political question of autonomy. We might some day have light terminals without storage or even serious processors with all the data and work still done in our cellars garages and and attics via 8G or whatever grade connection. If there will be enough demand for market and politics for devices to be available, of course. So yeah, I think, culture and politics hold the answer.

I was lucky to find https://purelymail.com/ which fit my budget and needs - primarily supporting multiple domains for my family and personal projects without extra costs.

You might get surprised - I looked at default KDE, Gnome and docs for a couple WMs. Yet my question is not “how do I get different backgrounds”, but rather, “how come is this the usual thinking”? Why? Are there serious drawbacks technically or from UX perspective that I didnt think of?

Why downvoting? It is good for community to know this, even if content is far from cheerful. Expressing criticism in comments does more for public awareness than making the message less visible.