ᐅ Is the underlay membrane in the roof still durable?

Hello,

we purchased a house built in 2002. During the inspections, including one with a building surveyor, we accessed the attic space, but not every corner.

A brief explanation:
The top floor is a large room constructed with drywall. Behind it, the roof area is accessible. However, the remaining concrete intermediate ceiling is covered with glass wool insulation. Due to the roof shape—a bungalow with a bay window—it is densely framed with rafters.

At the inspection, we had already noticed that the roofing underlay won’t last another 25 years. The seller insisted that the purchase contract include the note "roof underlay partially no longer tightly stretched." Fine.

Now I have explored many corners more thoroughly and was shocked to find severe deterioration of the roofing underlay (photos attached).

What do you think about the condition and the risk of moisture damage to the insulation, drywall, and so on? Within what time frame do you think action is necessary here?

Thanks for your opinion!