I'm surprised that so many people still need help understanding the fundamentals despite it being a much-discussed topic.
Apple has no choice for multiple reasons.
First and foremost, Apple can't license the patent from Masimo. Masimo isn't interested in licensing the patent; they're only interested in a proper shakedown.
Masimo doesn't outright license its patents. Ever.
Masimo only licenses its patents if you also agree to a supply agreement in which they supply the components for you to use and with the strict terms that you use their components and only their components for implementing their patented technology.
Masimo's CEO euphemistically calls this “work[ing] with them to improve their product.”
Apple is not interested in that at all. It would be bad enough if they had to purchase a mountain of components that would collect dust in a warehouse; it's an entirely different thing to have to use those components that, from Apple's perspective, are not only inferior but not even suitable for the Apple Watch's small form factor.
Apple also can only do something once the pending cases have been resolved. Yes, you've read that right; there are multiple cases.
There is a pending case in the District Court for the Central District of California that was going on before the ITC case, Masimo was getting their ass handed to them in that court, and all but one juror was in favor of Apple. But you need a unanimous decision, so it was declared a mistrial with the intent to retry it. Massimo saw the writing on the wall, so they initiated a case with the ITC to try their luck there, stating that "it took too long" to do it through the regular court.
There's also a case Apple started against Masimo for stealing and using trade secrets to create their competing smartwatch product. These trade secrets were gleaned from internal documents Apple had to provide during the discovery of Masimo's case against Apple in California.
Now, there's also the appeal of the ITC decision. The ITC isn't a regular court; it's more like an arbitration process run by a federal agency.
Until (some) of these cases have been resolved, Apple's hands are tied.
There's also the matter of Masimo acting like a patent troll. Masimo consistently sues companies, usually competitors in the same field, while getting numerous patents invalidated to extract payment with the patent + component scheme.
I've lost track of the exact number, but in their efforts to go after Apple, they've lost some odd 16 invalidated patents. In 2023, Masimo got a patent invalidated while going after their competitor, Sotera Wireless; this is after Sotera went after Masimo in 2020, which was in response to Masimo going after Sotera in 2019. Every step along the way, Masimo lost a handful of patents.