Inertia was an issue, now it's solved with grid-forming batteries that can provide the same inertia a rotating flywheel did.

Most projects today are solar+battery or just battery alone, so inertia is no longer a blocker, it's just part of designing the project right.

Here is a "Practical Engineering" video on the topic, mentioning that your concern is real: https://youtu.be/7G4ipM2qjfw?si=qfVymRpKFpuexQF_

And here is a more recent video about how Australia runs on a ton of solar with no issues thanks to grid-forming inverters: https://youtu.be/qavFbOpt4jA?si=dlkEEN4sZLCv2os5