"I mean - there's a reason I haven't been complaining about the Smashing Pumpkins that show up when skilling"
Good then! Why don't you just take all the free H'oddments and whatever RNG based prizes including the 1 bil gp worth new mask you can get? You will absolutely guaranteed to get some cosmetics from the H'oddments Store.
"nobody is going to gripe about Seren spirit-esque pop-ups. They're freebies and aren't terribly invasive at all."
Totally wrong! Seren Spirits aren't freebies. You will need to be wearing a Grace of the Elves to get them and free players will never see them at all. Why should free players not gripe about Seren Spirits they will never get but H'oddments they can get from the Halloween events?
"There is no need to force players to play the game for multiple hours to obtain all available rewards"
Nobody forced you to attend a pumpkin party just like nobody forced you to mine a Shooting Star. You have been told numerous times we only take like 30 minutes a day to get 100 pumpkins from pumpkins parties. We can all see and experience it in the game, RS3.