You want peaking power for heat waves. Baseline is in theory cheaper because it’s running 24/7/365 however you don’t want to produce maximum heat wave energy 24/7/365 or your going to damage equipment when demand lowers.
Nuclear designed for load following vs base load has real physical differences with base load being cheaper to construct as a trade off for being less flexible. At the extreme end it takes several hours if not days for some nuclear power plant designs to safely go from standby to full output.
Peaking power on the other hand is designed to go for long periods without being used. They generally get paid for being standby capacity even when not in use. Similar to a generator at a hospital, they need to sit unused for months only be tuned on in at worst a few minutes.
Peaking power is the economical option for heatwaves or other extreme power demands. At the extreme it might actually be a backup generator pulling double duty. So what if it’s running on diesel if it’s only tuned on for 100 hours a decade fuel costs aren’t that relevant.