So if you go buy a new car you don’t drive it because the tires are going to wear out?…..My definition of “heirloom” isn’t an investment rifle but one to pass down because of what it helped accomplish, memories and meat in the freezer. As much as I would like to have a rifle in a cartridge for every need, I can only afford to have a few, like a plinking rifle (223) and a hunting rifle. My thoughts are to have my hunting rifle chambered in a cartridge that is economical and efficient in ballistics and fun to shoot. I have been shooting 300 win mag and 300 wsm for the last 10+ years. I love the power of a magnum other than the punishment on the shoulder. I have always handled recoil well but I know I shoot better with a less recoiling rifle. Not a fan of brakes and haven’t dove into the suppressor world yet. So I sold my 300 wsm and all reloading components for it. My “forever” rifle will probably be a non-magnum.
This actually puts a very different spin on things, OP. Knowing it's not a thought exercise about what would work best as a do-all, or issue of ammo availability, but is instead an issue of "what one gun would I love to hunt with that would be the coolest thing for one of my kids to inherit, because of what I've done with this rifle and what this rifle meant to me?"
That's actually a cooler question, IMO. And honestly makes me want to go kinda fudd, thinking of wooden stocks and somewhat obscure or less common cartridges. Not a collectors' item though - something meant for the field and modifiable as necessary, but of extreme quality in terms of both build and performance.
Something custom, with Grade 7 or better wood in a non-glossy finish with exceptional checkering, on a high-end action like a BAT Machine or Zermatt... chambered in something uncommon and bad-ass, possibly even something that you just can't find on the shelves. Maybe 25-06 Ackley Improved...but 280AI, .257 Weatherby or .270 Weatherby are all right in that ballpark for me. Probably topped with a Schmidtt & Bender Polar 4-16x or a Tangent Theta in 3-15x.
Add in a pile of brass, dies, my own reamer, and a few spare barrels already spun up for it by the original gunsmith, and I'd be a very happy man hunting with just that one gun the rest of my life.