3catcircus
Adventurer
Core mechanics are easy to convert. It's the mechanics providing setting-specific flavor that are the problem. Recall the 3e regional feats? Great idea to provide specific flavor. Now do that for Dark Sun or Greyhawk... I seem to recall a Dragon article that tried to 3e-ize all the old settings.We just ran through the 2e adventure(s) Four From Cormyr using 5e. Our DM converted on the fly, and the adventure ran exceptionally well. I'd never even heard fo that adventure before and it was a blast. Have you tried using 2e adventures in 5e?
Now what about the different cosmologies in the different settings vs core? I've handwaved it away by stating to my players that the Great Wheel is the true architecture and the FR or Eberron architectures are how people in those realms perceive it - frame if reference and all that.
What about cleaning up idiosyncracies between different products for the same campaign setting that don't align with each other?
All those things are what takes time - time that older players and DMs usually don't have. No DM wants to think through 4 different references if that don't have to. The 3e FRCS as far as I'm concerned is the gold standard for what a campaign setting book should consist of, if it is given a 5e treatment.