On machines four or more years old with heavy duty cycles, replacing the thermal interface material has produced a clear improvement in sustained behaviour in my workshop. On two-year-old machines it has usually produced nothing I could distinguish from run-to-run variation.
I want to be careful here because this is exactly the kind of claim that gets repeated as fact. I do not have paired before and after measurements under controlled conditions. What I have is a consistent workshop impression across perhaps thirty machines.
Treat it as a reason to check an old machine, not as a recommended maintenance interval.