I never understood the people getting tired of it, really. I got mine for 1600$ and I just knew how good and what this lens was about after I shot 1 single roll with it. Anyone who understands what it takes to create a Noct and what kind of glass and engineering it takes will realize it whole potential.
The real problem is the people that don't have a clue about photography, those that get into the Leica bandwagon to imitate HCB and that quickly get tired of it. Those are the same ones that snobbed the Nikkor 28 1.4, The Noct-Nikkor and other gems while they we're still affordable.
The masses.
This being said, it's all my own opinion. And while I understand the Noctilux' price going up because of what it takes to produce it (no, it's not speculation as some people think!!!), I still don't think it is an investment simply because lenses are meant to be used.