There are NO suitable replacements.
NAC tried to use the ATR, but it just didn't fit the needs of the carrier, and they got rid of it after just a couple years. It's still flying, in Europe I believe, in NAC's colors.
The DC-6 is a monster, the C-46, half a monster.
Because there are no real replacements with similar characteristics, the business model will have to change, IF
these birds are to be done away with. NAC has converted to a fleet of 737-200s and 737-300s. Everts could do the same, but likely won't, if you ask me.
-80s will be a good addition, but they cannot do the same things at the same airports as a C-46, that's for sure! They're designed for entirely different missions.
You can't park a tour bus in the same space as a Smart Car; no one ever intended you to. So if you're gonna start operating a tour bus, that's fine, but you're going to have to adjust your business accordingly.
I hope that poor metaphor helped explain my point!