The earliest satellite in the constellation was launched only 5 years ago and is less than half way through it's minimum design life.
Yes failures have so far not knocked any satellites off line but only because of redundancy. There is no information as to if these failures are random or systematic. In either case the redundancy comes into play to prevent well controlled random errors most likely to occur as satellites approach end of life.
A 12.5% failure rate in the core components of a system that has been in place for less than half its design life is alarming by any metric. All the rubidium failures have occurred in satellites that are less than a quarter of the way into their design life.
On the upside, given the go live dates under EU law the rubidium clocks are covered under manufacturer warranty. But the shipping costs are going to be a bastard!