It’s not semantic, that implies it’s just a name difference. it’s economic. Base and backup aren’t used in the same way and they have very different economic models. Eg you run a backup gas peaker very rarely and charge a fortune for the power provided during those windows. Base relies on you selling every day, all day for a cost competitive with other sources.

And nuclear can technically be ramped yes, but not economically. it has to run and sell at 100% 24x7 of the time to make the economics of a nuke plant work. The enormous capex and high fixed opex make nukes terrible as a backup power source

You are right that most modern grids do still have and depend on some legacy fixed supply, eg old nukes, non-peaker gas, or hydro. That is changing fast though, as batteries will eventually push cheap solar through the night and eliminate that economic niche too.

This isn’t theoretical - look at Californias grid. It’s going to exit 2026 at roughly only 20% fossil (gas). As the massive battery and wind rollout is covering more and more of the nightly part of demand. If they keep it up within the next 5-10 years there will be zero fossil on the grid in a regular daily cycle. Leaving gas as a true backup for weather events and suchlike.