FWIW I'm having one of my rear Michelin PS4 replaced on Monday. Despite it still having over 3mm tread it's nearly 7 years old, has micro cracks, a sawtooth pattern on the inner shoulder and it's getting noisy like a worn wheel bearing. Not taking a chance as I'm doing a continental road trip next month.
I should really swap the tyres between front and back once a year to even out the wear.