I’ve had Michelin tires on both our cars for the last 15 years. They normally last about 65,000 miles or so before the treads wears down. I have had only one (1) puncture, which did not result in a blowout, in all these yaers.
In all cases, when replacing the tires, there were NO CRACKS and the tires were pronounced SAFE by the vendor, usually a reputable firm. You are needlessly scaring the OP. If he had $40 tires from an unknown Chinese manufacturer I would agree with you.
In this case the mechanic is right and I would comfortable drive another 5000+ miles on these tires, provided there was no mechanical damage for sharp objects.