Impractical, for many reasons.

Ignoring the bit where the community is expected to come up with choreography to handle mechanics on their own, the NPC action scripts can't handle the variability of choreography that human players can. For example, as a RDM I sometimes need to be in melee range at specific times and I can arrange with my party for that to be okay, but a full party of NPCs would expect me to take a ranger or caster spot every time. And if the choreography used by the NPCs is less efficient or harder to execute than the prevailing PF choreography, you'll never be able to apply what you learned with the NPCs to a real party or vice versa.

If you died, even if the fight didn't end immediately as it does in current Supported duties, the NPCs wouldn't handle it anywhere near as effectively as a human player. They might raise you in a spot where the floor is about to disappear, or they might get caught by the floor disappearing while they're slowraising you.

And then the matter of damage output: the NPCs would have to be able to put out enough damage to clear the duty, and if they're putting out too much damage then the player gets a free ride, and if they're not putting out enough damage then only high percentile players will be able to clear with NPCs, and those people are the ones who probably don't need Duty Support in the first place.

There are lots of reasons we won't ever have Duty Support in high end content.