I do not pretend to understand thermal expansion and all of the technical stuff you are talking about, but outside of an errant hockey puck from my son as a youngster, in 35+ years of home ownership, I have never had to replace a door window. That includes storm doors and prime doors; wood, aluminum and steel; full glass and partial glass.
Either I am very lucky or you are doing something wrong.
This is an interesting problem. I can offer a comment, which is that companies that produce consumer goods like this really do not want to hear about how to solve problems or change the product. The engineers on staff are there to keep production moving, not to change the product. Not only are they too busy to evaluate a change, they probably do not understand what you are talking about. My suggestion is to find a source of aluminum extrusions that can replace the molded plastic piece, take the door apart, as much as you can disassemble the window part, install the aluminum extrusions using a urethane construction adhesive, and then reinstall the window glass. A heavy bead of the adhesive should allow adequate differential expansion and contraction.
Probably, though, part of the failure is due to the dimensional change of the plastic as it ages. The result is usually a small shrinkage of around 1%, which does not start until after a few months. Since the plastic surrounds the glass, which may expand a bit as it heats, while the plastic shrinks as it ages, the plastic fails in tesion, and the gaps appear. As the aged plastic cools in the winter it shrinks more, which opens the cracks more.
So my assertion is that the root cause of the problem is a very poor choice of plastics, coupled with poor manufacturing techniques. And an "engineering" team that was pleased to get such a good price reduction.
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