AD is better than main stat simply when you go for the high GR pushes and only play dense rifts (and rift guardians with adds). There are two scaling factors at work here:
- Main stat gets less efficient the more you have (sort of a diminishing return effect). The more main stat you have, the more interesting are other stats.
- The usefulness of area damage purely depends on how many mobs are around you. If you play every single rift to the end, you would never roll off main stat to AD in your example. The more selective you are with picking your rifts, the better will AD be.
There's no general answer to this question because "it depends". You could probably take some example numbers of main stat, AD, number of mobs around you and then put it in D3P (simulation mode). With a bit of effort you could even create a plot of all those to extrapolate "which is better when". But you definitely have to take "anticipated number of average mobs around you" into account.
If you pick bad fights with less than 5 mobs around you and kill every no-add rift guardian, main stat > AD.
If you only play 1 in 100 keys because you're pushing for a new record, AD > main stat.
For the more realistic examples in between, "it depends".