I agree completely. Reasonably good thermal paste properly applied does not "wear out" for a long time. It probably will dry out and become rather hard, and that makes it difficult to remove after several years. But that does not make it perform poorly, so age alone is not a reason to replace it. Performance is how to judge. That is, are you able to continue to cool your CPU properly?
You cite a concern about recent higher temperatures. There are two likely causes for that, that do NOT need replacement of the thermal paste. One is the accumulation of dust that you cleaned out. IF you got it all, the temperature situation will have improved. However, one place that can be hard to remove dust is the spaces between the fins of the heatsink attached to the CPU. That area is under the CPU fans and hard to see and to reach. But if those spaces are filled with dust, the air can't flow through there and help cooling. The fins of the heatsink are supposed to have clean empty air flow channels between them. I saw a photo in a post here some time ago showing a heatsink with all its spaces tightly packed with old dust and it was performing very poorly. So, how to check? Look again inside your case and try to see through the CPU fan to the heatsink fins underneath. If you can see clearly that the gaps between fins are pretty clean, don't worry about this item. But if they are filled, you need to clean them out. Do NOT remove the entire CPU cooler system from the CPU chip. The fan alone usually can be removed with some clips or screws that fasten it to the heatsink. Once it is off, carefully use a thin tool (toothpicks, nail file?) to loosen the dirt between the fins so it can be blown out. Don't push really hard - you do not want to disturb the CPU in its socket, although it is held in very well. Once it's cleaned out you can re-attach the fan.
The other factor to consider is your own use pattern. Compared to when you remember the temperatures were lower, do you now use more demanding applications and games? These might increase workload and hence the CPU temperature.