Re: "We improved [..] Galileo purely by retuning the software in the ground segment."
Complex systems will inevitably degrade or fail, particularly if they're exposed to the elements (or space, as in this case).
The goal is of course to make them as reliable as possible, but it's far worse to sustain a failure and not be able to detect and report it.
Aircraft have many layers of redundancy, a single GNSS failure is not critical under normal circumstances. Even if all satellite navigation equipment fails, it will be perfectly possible to continue flying, but it will certainly make it harder to navigate. At least for now - traditional ground-based radio navigation stations (such as VOR/DME) are increasingly falling out of favor.
We'll see if recent GNSS jamming events can change that, so more resilience is built.