Schrader- or Presta-Specific Rims Going Extinct?

Trek 830 Presta-only rim.

At one time there were Presta valve-specific rims like the one above. I loosened the screw so the hole is visible. As you can see, it's a tight fit to the rim.

Ross's Schrader-only rim.

I have Schrader valves and rims on the Ross. There's a wider hole in the metal to fit the wider valve stem. Again, form fitting setup.

Husband's new Presta/Schrader rim.

Now, when I went to the LBS (local bike shop) to help my husband decipher a problem with his freewheel, the diagnosis necessitated buying a new wheel. I knew my man wanted to go back to Schrader rims so I inquired about it. The clerk replied that there was only one size that fit his freewheel, but that if he wanted to later use Schrader tubes, he could now with the new set up. I asked how long those new rims had been available. "Oh, a few years." I scratched my head on that one, but made the purchase and brought the bike home. 

I decided to investigate. I loosened the valve screw to reveal a rubberized plug in the hole. With further Internet searching I discovered there is a Schrader to Presta rim hole adapter, which is apparently used in this situation. It's only a dollar. Now, whether one can refit this adapter to an existing Schrader rim, I have no idea, but if so that could open up possibilities. Aside from streamlining the manufacturing process (no more separate Presta- or Schrader-only rims) this universal product seems rather brilliant. 

This leaves me with all kinds of questions. Does this apply to only 26" mountain bike rims, or will this new technology apply to other sizes? How long will the adapter last and should one carry a spare? If anyone knows more on this subject, please chime in.