I think we are pretty much saying the same thing and you may have taken or I may have worded my statement slightly wrong.
Yes as the bulb dims the ambient light in the room from sources other than the screen will remain the same and the screen remains the same.
My point is if you are of the opinion that some of are that an ultimate shade of gray and also an ultimate sheen level can be selected for a given setup there is only one draw back. That being if the selection was to be made around the new bulb brightness then you could have a screen that would become less than ultimate at some point in the life of the bulb. That's why I have cautioned people from going to the darkest of grays in the beginning even though the results at that point in time are better than something lighter.
If a screen isn't spot on neutral you can adjust for that and that won't change much from then on. Brightness has to be looked at as the biggest variable of change. Most bulbs take a hit after a few hundred hours and then hold pretty good for a long time. I kind of charted my brightness settings over time and at 200 hours I made a fairly good adjustment, each 200 hours after that the changes were small but always one or two clicks. After doing each adjustment the image was exactly the same as the first time.
In these last experiments Todd did the BW started out stressing the light limits he had, and you would expect the strain of those limits showed most in the darkest of images.
You are right as the brightness diminishes with a bulb the white end and the black end both come down together. Do they come down equally? I would think they would. Not really different than a projector with an iris to adjust or one with a ND filter.
There is no variable ND screen surface yet, one that would get lighter as the image gets darker to always allow the best shadow detail possible. That's why we need a screen that's been selected such that the projectors brightness can always be adjusted to give us the full gray scale range between white and black.
I always tell people if you only do one calibration do the one that sets the range from black to white. The rest can be done with filters or by eye to taste and most people will be happy.
One last point (at least in this post) on gray scale selection. Conventional wisdom recommends gray screens for ambient light. I have never believed that and I see reason for using them under mixed lighting conditions, as I do in my viewing room. If you wish a screen to be used under 2 or more ambient levels of light then the gray should be selected around the highest ambient light you have and then compared under lights out conditions and also recalibrated. Just so you know going in the overall brightness will drop as a bulb ages and you will be able to maintain the PQ over the life of the bulb with both ambient levels. What I have found is if you have the lumens and the dark screen to work with the ambient if anything you might be too bright lights out. And that has as much to do with our eyes as anything.
Oh there ain't no bugs on me
There ain't no bugs on me
There may be bugs on some of you mugs
But there ain't no bugs on me