It depends to some extent how much can be updated versus starting from scratch and building again.
I'm currently updating one of my very early routes to run in TANE and, while it's surprising what a difference replacing terrain textures and updating to Protrack makes, it still remains a 2005 route in most respects. For a shorter route, it might even just be better to start over from scratch. With a constant flood of new ideas and limited time available, there's also a balancing act between how much is divided into fresh projects and refurbishing the old.
This also pre-empts any decision or debate with N3V as to how much backward compatibility TS2018 will have and whether that, in fact, will hold back the possibilities if it still has to cater for UTC era assets.
If it truly as easy as N3V portrays, then your route could easily be "brushed" over with new clutter meshes, and hopefully brushed with the Turf Effects as well. To me what mostly sets apart an old route from modern is the amount of trackside detail. For example just look at the S&C route from 10 years ago, compared to the new route released by Rumour3. The amount of trackside grass makes a great difference, as it gives the impression of detail that may not have been possible 10+ years ago, and will be easily possible upcoming.
One of my plans is to update the S&C (for personal use) somehow, with the new features. Of course the Protrack upgrade, and maybe a good number of updated stations and buildings. More trackside infrastructure (signalbox linkage, signal wire, trackside relay boxes, etc.) but this would be impossible without an idea of what the route looked like in the early 60's. Definitely new clutter and Turf Effects, and what else N3V have in store for us!
I think the problem with cutting off backwards compatibility is that it means a solid year's wait for new content. The built-in assets are not enough to sustain everyone's needs, and it's getting more complex, tiring, and frustrating to create new content, which is why creators seemed to have disappeared.
To combat this, N3V could dip into their revenue and commission some new projects? It would no doubt be expensive, but it's a matter of what coveted content does N3V TS2018 offer that DTG TS2018 doesn't?
British Railways is extremely well-represented in DTG's game, while in Trainz it's more or less represented, but getting longer in the tooth! However much N3V splurges on commissions, I think they could easily recover that money through customers buying their game for the chance to play with some new content.