First of all, I don't see that happening. I don't see how it would be possible to make an all polymer gun.But, you're OK with totally polymer guns?
If it were possible, or becomes possible sometime in the future, you can bet that I wouldn't be first in line to buy one.
I think that it's not really profitable to worry about guns becoming all plastic just because some gun parts are now made with, or partially out of plastic. One might as well worry that cars will soon be made entirely of plastic because plastic is now used in some car parts.
From what I can tell, in both cases, the choice to make certain non-stressed/low stress parts out of plastic in both cases has not resulted in products that are less serviceable when the manufacturer has a dedication to producing a good product. It may be esthetically unacceptable/undesirable to some, but when it's done right, it doesn't seem to cause functional problems.