All things cycle
Yes, the current era of 'surplus' will end. One doesn't see old Webley revolvers sold in bulk like they were in the period immediately after WWII.
Depending on political control and market pressure, there may be 'surplus' M9 pistols at some time in the future. On the other hand, Beretta probably wouldn't be happy about that, as that might cut into sales of newly made pistols. Of course, they might make a killing on spare parts. (Who knows?)
The biggest problem is that of fully automatic weapons. In order to 'surplus out' old M16s, the U. S. would have to repeal several current federal laws. It's possible, but not what I think 'likely'.
However, on the bright side, modern manufacturing methods can and probably will drive down the cost of new firearms. We can hope.
Stick around and find out, eh?