Very good points, thanks for the reply!
Some things might be inherently pleasurable, or rather are pleasurable just because of the qualia
I completely agree! However (and perhaps I misrepresented my own view here by making things too simplified and black/white for the sake of comprehension) you still have contrast. For instance, an orgasm would likely always be more pleasurable than eating a piece of toast, regardless of whether or not you could experience any sort of displeasure. My argument was more of, "If an orgasm is 10/10 on the pleasure scale and everything felt like an orgasm, it's the same as being 0/0 because there is no longer a scale." Of course the counter-argument here (not much of an argument really) is "Everything would just feel awesome!" Unfortunately, that assumption is made from within the context of a life that does have a scale, so it's meaningless.
Since I can't rightly say that the unknown is finite as I have no idea, I won't argue that eventually Einstein would become so bored that he wished to die. However, I stick by my opinion that if life where eternal it would most certainly redefine the idea of life as we know it and would likely ultimately dilute the experience. To visit the orgasm analogy again, I think life is sort of like one -- a very brief period of heightened pleasure, awareness, etc. If you remove the brevity from it, you eventually remove the pleasure.
I think most people would agree that life is about the moments. The orgasms, the successful companies, the broken hearts. We define our lives based on these moments, we don't define them based on the dull in-between; eating breakfast, taking out the garbage, brushing our teeth. Many of us, I think, strive to fill our lives with as many positive moments as we can mostly because (consciously or otherwise) we realize that we have only so many moments to make before there's simply no time life to make them. If you could live forever, would you honestly work so hard all the days of your long life to make those moments? Or would the lack of urgency dilute your desire for that? Even cause you to eventually lose that desire entirely?
Hell, I don't know. I'm just of the opinion that it would.