You are right in that there is no such thing as "baseline" generation really. And thermal plants also need to be over-provisioned.
There is dispatchable and non dispatchable power, the former being plants that can alter output on demand.
Base-load is the point which the 24 hour cycle of demand doesn't go below. It's pretty much just a number.
Who occupies the base of the generation stack and will stay running during low demand, is the one who is cheapest. If wind is cheaper than nuclear, it will displace nuclear. The people who say "IMO we need nuclear for baseline and renewable for peaking" completely miss these facts.
The way it is going is renewable at the base of the stack and dispatchable power filling in the gaps, be that during peak or periods of low renewable output.
Very good idea to run gas plants with 490 grams CO2 per kWh instead of nuclear power plants with 12 grams CO2 per kWh, indeed.
France produces 50 grams of CO2 per kWh while Germany has 400 grams on average:
> https://www.electricitymap.org/
Your ideal model for producing electricity actually hurts the environment as the times where gas plants need to run are much more often than you think.
A 100% nuclear grid will need dispatchable peak power as well, be it gas or storage.
Put another way, it's probably more likely that they'll be some sort of equipment malfunction than we go an entire month with no wind.
Providing redundancy through independent mechanism (over-provisioning, storage, diverse sources) helps a lot.
But you're right. A grid that's 99% renewable with a natural gas peaker providing that last 1% will be a lot cheaper than a grid that's 100% renewable.