Very much agree with that point and that is something I have done multiple times when receiving excellent service and notably when they take initiatives that go beyond the airline's standard guidelines to improve the experience of their passengers. I once ended up with an inflight lead (on AF, not QR) in tears when I boarded ta flight. I had sent a thank you message detailing why she had been such a fabulous inflight lead a couple of months before, and not only had this made it to someone in the inflight experience team who had forwarded my message to her (it was anonymous, but some of the details undoubtedly made it clear to her which flight this related to and I had already thanked her emphatically when disembarking) but this had been used as an example of good practice in one of the regular crew training sessions she had been part on shortly after.

Obviously, I had no idea they would make such a fuss with my comments as I'm sort of used to how negative comments tend to get copy and paste empty words and "we'll tell the relevant services" which they never do, but many airlines have processes to systematically forward positive feedback to the crew member concerned and their line manager (I know BA do it for instance) and also, there are times when inflight experience teams are serious and keen to improve customer experience (it was shortly after AF had fallen very low and before they upped their game significantly with the results we know now such as a pretty great First, etc) when they do, they can seek positive feedback and analyse it.