> old mature

Certainly it is old and mature: it has been optimized to the max because it is in so many power plants and industrial processes. Don't expect some large improvement in steam tech.

> therefore unscalable

Again a matter of steam and temperatures and cooling etc. Scaling nuclear large is less of a problem than scaling small. I doubt SMR nuclear will go very small or be successful. Whereas some renewables scale down to the level of a single house.

> What do you specifically mean with the generalization "renewables?"

The usual definition: solar, wind, tidal, wave, geothermal, hydro.