I'm not a particular fan of turning investigative services over to the RCMP. About 70% of the RCMP are street cops--municipal and provincial police--which is a growth industry within the force while federal policing is declining primarily due to funding problems. I would expect that the RCMP would definitely seek funding from DND to cover any policing services it provided. The result is that while many of them are experts in quite a few categories, few would have any greater experience at the type of policing that NIS does than NIS itself.
Not sure you’ve got the right read on the future of the RCMP. While there’s some growth in contract policing (municipal/provincial) simply due to population growth, that’s not the RCMP’s future. The writing is very much on the walls. Until recently the RCMP were getting paid something like $15k a year less than most other police. That’s no longer the case. While the feds still subsidize RCMP contract policing, it’s now a much smaller margin. The RCMP’s largest municipal contract in Surrey BC is turning into a municipal police service as we speak. That’s >800 Mountie jobs disappearing. Alberta and New Brunswick are both talking about or actively exploring provincial services.
Conversely, the federal mandate is only getting more and more complicated. Cyber is a major growth area (or is trying to be). Financial crimes has been inexplicably left to fester, but is now a bigger priority again. National security definitely isn’t getting any prettier. COVID has stopped Mounties from being released from provincial contract policing, but that will open up again soon.
Most criminal acts committed by CAF members can easily be absorbed by the civilian justice system and local police, be it Halifax, Victoria, or Edmonton police, OPP, Sureté du Quebec, or local RCMP detachments. For the relatively rare cases that would require overseas travel to investigate something in theatre/host nation, there are Mounties getting on planes all the time. There would simply need to be appropriate statutory authority to investigate and prosecute extraterritorial offences (the Crim Code already has such a provision for public servants in the course of their employ), and some unit(s) in the RCMP would need to assume this mandate, presumably with funding for a few more bodies.