Thermal paste doesn't dry out. Period. That's the whole point of thermal paste. On top of that, ideal conditions for rack servers (which are way higher load than home desktop boxes) are nearly bone dry. As for heat, it tops 95F for hours at a time in high summer in the room where I keep my box. Europeans don't generally have air conditioning.
Unless you're living in the Mojave and sand has been getting into the case, there is really no reason for the repair folks to be replacing parts on a yearly basis unless they're bilking you.
On the contrary, if they're swapping heatsink fans or other such things that often, I'd be worried about the wear and tear on the motherboard itself. And that's expensive to replace.
I'd rather use the money for stash... Becky A.