The Sum of Awe
Brought to you by the moment that spacetime began.
Let us make this simple : Prove it. Show evidence that living too long in an afterlife is inherently a bad thing, and then we can continue our conversation.
Penumbra has the proof.
There is of course a difference between perception and reality.
If I really like cake, I could imagine that I'd love to eat a whole cake. Prior to eating the cake, if I am inexperienced, I may think eating a whole cake will be wonderful, and define it as a happy experience. Somewhere around two slices in, however, I'm going to not want to eat cake anymore, and realize that my initial perception was incorrect. As surprising as it may be, there's such a thing as too much cake.
When it comes to afterlives of infinite duration (rather than afterlives of timelessness), this is basically the problem that's occurring. An overly simplified and usually poorly defined promise is being made- life that never ends, and many people think that sounds great. But much like the cake, there's a difference between perception and reality.
Arguments like the one the OP is making point out that, logically, a lifetime of infinite duration should eventually become unbearable. Just because it is defined as forever happy doesn't mean the idea itself has enough substance to support such a claim.
Basically, like the cake. It looks so delicious, tastes so delicious, you would love to eat 10 of those things... But the fact is you'll get sick of it.
You'll get sick of existing.