9 year old tyres would also have been dangerous. The compound as you observed with the new tyres had hardened on the old ones.
I usually change them after 6 years
I also change tyres at 6 years, enjoying the subsequent improvement in grip and ride comfort.
Any TMers who have ridden motorbikes will be very aware of the reduction in wet weather grip, as the tyres age.
I am aware of a Morgan owner, with a very low mileage 18 year old car, which (he proudly states) “still has its original tyres”

Bud
4/4 - Stanley