I think there will always be specialists doing things that GPs can also do, for a couple of reasons:
1) Some GPs just arent comfortable doing things that they didnt do a lot of in dental school, and theyd rather not deal with the added stress. For example, some of my classmates and even faculty Ive spoken with didnt get a chance to do much difficult endo in school, or had very bad experiences with it, so they just arent comfortable doing it out in practice. Theyd rather stick to the bread and butter restorative and become very good at that aspect of dentistry.
2) It may not be financially worth it for some dentists to do certain procedures like implants, molar endo, etc, just because the added financial gain from doing these procedures is not worth their added cost. It could be real monetary cost on overhead, opportunity cost of spending time doing implants/endo while you could be doing crowns (or other things that you can do faster), or even the perceived cost of worrying about being sued, etc.
One of my relatives is a GP, and she doesnt place implants because in her mind, she can spend 30 minutes restoring an implant and charge $1200, or she can place and restore the implant for more money, but with the added time, cost, and headache.
I do think that more GPSs will be placing implants in the future, but not at the expense of OS or perio, but just because implants will be a lot more commonplace.
My 2 cents.