Sorry what is your issue here? Arceus being essentially a bonus reward for completing those requirements is pretty nice. Calling it a paywall feels odd when Arceus in BDSP is bonus content and not required for anything. It's just a shiny trophy.
(Besides it isn't like you're paying for arceus only. You get a full length game with it. Which further makes it more of a bonus than anything else. Much better than having to go to some super specific irl event or miss it)
My issue is in the past you didn't have to shell out 60 dollars for an entirely different game to have access to events. They were released via mystery gift or at worst through physical locations like Gamestop or Toys R Us.
BDSP isn't even a good game. The least they could do is just give you the Arceus ticket like most other mystery gifts did. Pokemon games used to be pretty self contained but now we have DLC, paid subscriptions, paid online, and now even events are hidden behind another pay wall.