Flat roof 20 years old - replace now or wait for it to fail?

UK end terrace with garage and kitchen extension making for big chunk of flat roof round side/back of house.

We’ve lived here 15 years. The survey when we bought it referred to the garage/kitchen roof as needing replacement within 2/3 years. It must be at least 20 years old now.

A roofer looked at it 18 months ago (house insurance required flat roof survey every x years) and the report said “…cant see any foreseeable problems arising in the near future”. Verbally he told me it would last another 20 years but that was probably tongue in cheek.

The roof is fine insofar as it doesn’t leak, no pooling water, no sagging etc.

We’re planning a garage conversion. Im interested in whether we should replace the garage roof at the same time or wait for it to become a problem. There’s an argument to do it now, but surely an argument that doing it before it needs it is wasting money?