Just going to throw in my weight with the others, in that I just don't see this being a massive problem.
I think that any engine upgrades - if DS do in fact do any - will come down to something other than the need to be on the newest, shiniest graphics.
For example, if SpatialOS grandfathered out an whatever version of Unity Fractured winds up being built on? That would be a huge deal... And depending on expectations set between DS and SpatialOS, that alone might be enough of a justification for regular engine upgrades to make sure they gap between the current version and the latest SpatialOS supported version never gets too large.
Because it turns out you can accrue Technical Debt even if you don't touch anything! In the event that the market moves forwards multiple versions you can lose access to formerly viable upgrade pathways, and then when all of a sudden you have to upgrade you're stuck on a smelly waterway without a propulsion device. What fun! (my company has going through dealing with that very problem for the last 18 months and we're still not done...)
So there may still be really good reasons for DS to keep Fractured up to date on current or semi-current builds of Unity. I just don't think that being on the latest version on release simply for the sake of being on the latest version on release is a powerful enough issue that it's worth the time and effort to solve.
If the engine is a year or two out of date on release, I'm not personally going to be too fussed. If anything that might even be a plus given not every system may be able to run the bleeding-edge version or it might still be a bit buggy. But a version that's a few years old should have had time to propagate around the marketplace and should have had most of the most obvious bugs and issues killed off by then.