One program with different tiers of access seems just as confusing, if not moreso, than just having two distinct programs. There are plenty of people in the US who want Global Entry but have no intent or desire to go to Canada. Some may even be ineligible for Canadian entry without significant additional paperwork (e.g. DUI conviction, which Canada handles much more seriously than the US.) It would be nice at times to be able to get the Global Entry privileges while waiting for a NEXUS interview, but ultimately I think it's clearer to keep the two programs distinct.

If anything, at least from an outsiders' perspective SENTRI seems like a more odd program to break out from Global Entry. AFAIK the US is the only government involved in both programs, a Global Entry member can register a car to get SENTRI privileges when crossing the border, and so it'd seem simpler to just have both under Global Entry with a requirement to register a car when using the Mexican land border by car.