Inverters can easily replace physical inertia, it just requires technology developed within the past 30 years, and most grid folks haven't thought about new technology for far longer than that.
As more and more intermittent renewables get pushed onto grids, they become more reliable. Most outages are from single points of failure from large generators or transmission. Dealing with highly distributed renewables means that grid ops get used to acting fast, and there's greater redundancy instead of so many SPOF. Kind of how cloud services got reliable by expecting there to be failure and designing it into the system.
Storage is advancing super quickly, is super fast to deploy, and can replace a lot of more expensive things like transmission upgrades.
We have all the tech to replace fossil fuels on the grid with the above. The only question is the final cost. It's likely to be far far lower than using existing "hard" energy, because by the time we can deploy 50 TWhs of storage, it will have gotten so cheap. We don't know when costs will stabilize, but they have a loooong distance to fall.
And we have all sorts of other technologies that will make all this far cheaper: enhanced geothermal, enhanced geothermal with temporal storage based on injection pressure and release, iron air batteries, flow batteries, thermal storage for industrial process heat, etc. etc. etc.
For every area of the energy economy, there are two to three solutions that look promising. Fusion and fission look promising for none. That's not to say that they can't have some serious innovation and start dropping their costs, but nobody currently operating in the field has demonstrated a path. Yet.