Legacy concerns

Do as many Dratini quests as possible :-)

It's possible, I don't think you have anything to lose by evolving your perfect to outrage. It already had it's CD.

In typical Niantic fashion, "We'Ll SeE".

Seriously, it's impossible to know what will and won't go legacy. your best option is to make sure whatever you're worried about gets the best moveset it can learn now in case it goes legacy unexpectedly. If it gets replaced with a better option you can just TM it. If it doesn't get a better option but goes legacy anyway then you've covered your ass.

You should evolve the perfect. DT/OR is one of the best movesets in the game, Dratini CD was already, it's very unlikely that Dragonite gets a new better moveset. If it loses DT/OR and gains a new slightly inferior moveset, you'll feel sorry you didn't evolve and TM beforehand.

Whoops, I’ve been maxing my 15a/14/15 lucky shiny dragonair...now 99 candy away from an evolve. However, I do have three maxed DT/OR Dragonites 3551 to 3559.

nobody knows. All I can say is that DT/O is a damn good moveset and new moves are unlikely to be better. So better do it now

Evolve at least one Dragonite and TM to Dragon Tail/Outrage. Between other current and upcoming dragons, you don't really need a full team of any one of them.

I have two Dragonites (Dt/Dc and Dt/O), a Rayquaza, a Latios, and enough candy to evolve Salamence when its community day finally comes around. I've been considering evolving another Dratini with better IVs than my Dt/O Dragonite, but haven't had a need for it yet.

I would be more worried about Machamp losing Dynamic Punch or Blissey losing Dazzling Gleam.

As for me, when gen4 starts I'm going to try to get as fast as possible the final evolution starters, in case there's a move that lasts for the first 24 hours so I could get a nice legacy mon (This happened with Stone Edge Blaziken back in the day)