I personally get confused when more than 2 levels are involved. I have to then decide how much light I want, which I never really know, so I always ignore all the levels and just go for min or max. Whatever low mode is, it's good for night time bathroom trips and whatever high is, it's good for everything else (speaking as a mizer owner). So to me the 3 levels of the Haiku adds one superfluous level that I do not need. Also, the UI choice of the Haiku will likely not work as well for me, as the light is no longer predictable for boneheads like me -- if the level it turns on at is the same as what it turned off at unless it was cycled, then I have to remember to set the light to whatever I think I'll want it to be the next time I'm using it. Wouldn't want to walk into a movie theatre, try to find my seat and blind everyone...
I enjoy having clicky lights and they are fun and exciting to use but I also do not trust them on some level, as I have had enough accidental activations and worse, several failures. I know the McClicky is a great switch, because everything Don works on approaches perfection, but my primary EDC is, and always will be a twisty. Since the McLux PD offers both momentary and constant-on via twisty, it offers the robustness and the idiotproofing that I need.
As far as the bezel I'd be quite pleased if the PD design remained the same as it has always been, with a threaded bezel ring, but I understand that the o-ring design has some advantages. I am not a flashlight designer, and I don't pretend to have the experience that Don has, so I believe the decision is best left to Don himself, since the bezel ring or lack thereof has little effect on how I use my light. I assumed that the task of making a PD XP-G would be as simple as designing a new reflector, digging out the old designs for the original Ti PD (which is timeless) and then popping in a new LED but if there is a plan to refresh the PD design then I will still be grateful that Don has decided to revive my favorite switching mechanism for one more go.
I don't really know how the PD's sell for you, but to my recollection the last wave of PD's sold almost instantly. While you may feel that the Haiku is a far superior design I am sure there are still many fans of the PD left. If you decide the PD is done for good, I will be satisfied to stay with old tech. I may eventually come around to try the Haiku but I honestly can't see how this timeless UI design can be bested.