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It takes timeframes on the order of weeks to change the output of a nuclear plant

Tell me more. Either you don't have a clue about what you are talking about OR I need to turn in my Senior Reactor Operator's license :-)

Nucs like 100% power from startup to refueling but they can change power fast/easy enough to load follow but there are other issues that make it easier/more economical for fossil and hydro to load follow. Yes we could have all of our generation by nuclear plants but some of the operators would be busy changing power.

It takes timeframes on the order of weeks to change the output of a nuclear plant

Nucs like 100% power from startup to refueling but they can change power fast/easy enough to load follow but there are other issues that make it easier/more economical for fossil and hydro to load follow.

The "other issues" are that the NRC wants extremely costly thermo-hydraulic and radiological studies for a series of scenarios. The cost benefit of running with less than the full compliment of steam generators is the best example. Furthermore, to load follow, the NRC requires the grid-operator in control of the device(s) sending the demand signals to the power plant to also have a senior reactor operator license which also defies the cost-benefit analysis. For these reasons, nuclear plants are put on as base-load units.

Boiling-Water-Reactors are particularly adept at load following, if the license would allow it because there is a strong feed back between turbine power and the steam pressure in the reactor vessel. In fact, it is because of this feature many of the first generation BWRs were sold. In a PWR, the steam generator de-couples the feedback a bit; however, the Navy uses PWRs exclusively and achieves power changes out of their systems quite readily.