I really enjoyed reading your perspective on this issue. I personally don't mind the idea of NRE, but I think I understand it differently from you insofar as I don't think the end of NRE necessarily equals the end of the period in which you are excited about your lover, just the end of the period in which that excitement still feels frenzied. To me, the idea of NRE is kind of comforting - I think a lot of people grow up with the idea that unless you're fucking each other against walls 24/7, there is something "missing" from your relationship. NRE kind of normalizes for me the idea that it's okay, and even normal/commonplace, for sex drives (and romantic frenzies) to waver, as well as (quite importantly) the idea that when you're first falling in love, your best judgment might be a little compromised by your fear (of, say, loneliness) or anxiety or whatever.

I like your theory about IFE, but I do think there is something to novelty, because (to me, at least) falling in love is like discovering a whole new world. Certainly, you're intimately focusing on it as well - but when you fall in love with someone new, you gain access to all these feelings/insights you often have never had before, and that - the feeling of discovery, or maybe even "miracle" - does create its own, unique form of energy.

That being said, I think IFE is something that often overlaps with NRE, and while you can never quite get the NRE back (by definition), the IFE rises and ebbs with the reasons, almost. In my own relationship, for example, I often feel wildly excited about my partner even after over two years (which, I realize, is not terribly long, but long enough that most people consider it past the stereotypical "honeymoon" phase). However, it's much more of a renewed appreciation for something I have already, as well as an ongoing joy over something I feel like we've built and something I feel like I can rely on/come home to. When my relationship was new, I felt like a kid walking into a candy store for the very first time and first learning what deliciousness really was - the novelty is exciting because you never realized, or never experienced, that deliciousness for yourself before. Then it does become "normalized," and the sort of precariousness giddiness you feel at first gives way to something even better: the idea that the candy will never go away :D

...also, WOW, I wrote an essay, I am sorry. Apparently I have a lot of feelings on this topic!