I think the training process is long enough for hedonic treadmill to kick in. Yes you can keep doing the same thing that makes LLMs happy, but no people usually won't.
What makes you have this impression? I'm not all that knowledgeable about evopsych, but my naive mental model is something like, humans experience joy/sadness when they do something that would've resulted in increased/decreased fitness in our natural environment (with the caveat that: evolution can't hard code for very specific events, so it tries to match the behavior I gave above, but has to use a set of much coarser grained heuristics (and happiness consequently generalizes in odd ways))
And there is a pretty exact mapping between reward and fitness.
Like for me to start to suspect that a system has something like happiness, the following three facts pretty much suffice on their own
1. The system is pretty smart
2. Has been selected to do well according to some metric
3. The system is "acting" in some "environment"