Why do these have to be mutually exclusive? As long as I get all the Pokémon at some point I'm not overly concerned about when it happens.
A super casual player who only played an hour a week might take 2 years to get most of one gen of pokemon, for recreational players like my wife it took her 4 months playing 30-45 mins a day with a couple of longer Pokémon sessions if we did a date night to fill her gen 2 dex.
For me it only took a couple months and now I'm focused on getting higher IV versions and collections stardust.
My personal feeling is the order isn't as important as doing things to keep players engaged. Be that special events, switching up raids, releasing new legendaries, or adding new in game features it's all about keeping people interested.
I will admit, I'm super interested in Mewtwo and the Gen 2 dogs, but I'm willing to be patient to get them if I have something to do in the meantime.