10 year vision? 10 year engine...
Seems likely that Destiny 2 will be a downloadable component for the existing Destiny framework, but I'd almost prefer to see them split off from that and give us a new "base".
They don't even have to use the same base engine, framework, anything. Just as long as the character data can be brought forward it doesn't matter where the client sources itself from.
This isn't the 360 era anymore where game updates had to live inside the game's base runtime and squeeze themselves in with clever re-use of existing code and assets. Bungie can just ship completely new builds out as updates.
And you can do engine upgrades by just using a leapfrog strategy. New build for 2017. refine it in 2018 while team builds up new features for 2019. sustain takes over, engine team works on engine 2021.
tl;dr dont worry, all the benefits of going to a "new disc" can be had with PS4/One updates as well.