Hi Ron,
I can think of at least two reasons.
Los Alamos is at about 7,320 ft and I suppose the air is very transparent.
I have used different shades.
Some of the free resin? filters were shade 15.5, which is too dark in a polluted city but maybe O.K. in a rural site.
It may be that Los Alamos needs shade 16.
The Baader Planetarium material that you use may be a dense photo material rather than visual. I think they made various shades.
Doubling up I could see no filament of a clear 60 watt tungsten bulb with welders shades 13 or 14, which bulb I still have.
With a safe filter visually the filament is seen but rather dimly with one safe filter.
As to lifetime.
Eclipse shades or eclipse glasses usually state 5 years and some have very nice details of exposures. Say one minute every ten minutes or something similar.
Generally exposures should be rather short.
Organic materials, and I don't know which are, probably deteriorate over time.
I think that a glass, certified welders glass, has a very long life, but I don't know what it is.
Stained glass windows have lasted centuries. But glass does devitrify, maybe depending how it was made and what it contains.
There are numerous available shades and glasses on the internet, but I would only go with established known makes and ones that have honest certified marks. But we know fake marks are numerous, so people should be careful.
Buying cheaply is rather silly when ones eyesight is at risk.
Projection is still the safest way when done correctly.
As to opaque material covering a large scope, with a small window of eclipse material, I would avoid plastics as these may transmit infra red.
I suppose that metal is O.K.