Again - not an attorney (and IP law is a deep, complicated area of law), but that's not my understanding of how it works. My understanding is a derivative patent can protect the improvements (but doesn't always - not sure how that line is drawn, but I don't fully understand how that area works), but wouldn't protect the original IP. Again, the system is designed to ensure the technology/etc. eventually makes it to the open market, so there isn't any meaningful "extending" of a patent.
This is why SawStop is layering on patents to their technology. The core patent is expiring, but the inventor has put many derivative patents on top, which makes it pretty challenging to use the now-or-soon expired original patent in a product.
This Reddit thread on the topic was really interesting/informative (and a bit of a dumpster fire at the same time).
IIRC, the inventor and patent holder of the SawStop technology was a patent attorney. He's been very aggressive with his patents - apparently he approached all the big makers to license it, but was asking for a huge price per saw that included his technology ($100 if I recall correctly). I'd definitely pay $200+ for a saw that included that tech vs. one that didn't, but that lump of cost on every saw just isn't going to work for big makers. So he started his own company and has filed a LOT of lawsuits over the years. Ultimately, it looks like that even with all the effort/patents/lawsuits/etc. the technology will be fully in the clear in 2024.