As a direct answer to your opening remarks, this is the where I place them.
If only a bolt action gun- 260 rem or 6.5 swede its a coin flip. Especially if you don't mind a long vs short action
6.5 Creedmoor-If planning (you say you plan for a long time) for bolt and a gasser anytime and if ever possible in future, this caliber allows optimum flexibility between all different platforms and is readily accessible ammo. This also allows, in the case you stop reloading, you can still get ammo for it. This round covers all bases and I see no downside to it in any way. This allows for less money spent buying different dies, casings, etc.. (One and done deal). If you can not tell this is my overall choice.
My opinion is that the 6.5 is the most verccatile bullet made, it won't dissappear from the market, and sense you reload it doesn't much matter what's on the shelves after you do your initial comparison as you mentioned earlier so it's nice to be able to pick and choose whatever you want.
Ar15- 6.5 Grendel is a great round if you happen to get into that platform, I would not hesitate to bank on it being around for a very very long time.