The common narrative seems to be centered on "Sony want your data". And whilst that's probably part of the story (marketing, cross-selling and retention being other parts), I highly doubt it's the main reason. I think the main reason is actually what they've stated:
PlayStation have a duty of care to their gamers, and they care about the quality of experiences on their network.
Helldivers is an online environment where humans can interact. And if you are the kind of person who cheats or misbehaves in those environments, PlayStation want to be able to exclude you from their entire ecosystem. They can't do that unless they know who you are and how you're accessing their network.
And PlayStation have a history of moderation like this - they don't allow unvetted mods into the PlayStation ecosystem (see: Skyrim / Fallout), to this day they have a highly trained and very active trust & online safety team and they have very strignent rules on user generated content across the network.
They've stated that this linking system should have been in place from the start, and that they removed it because it became a blocker to gamers enjoyment when Helldivers 2 was scaling up (remember the server issues?). I think it's a good thing that they were willing to be flexible like that. And I strongly believe that this would have been a total non-issue had the need to link your PSN account been there from the start.
What does matter now is that they make sure that everyone who has paid for this game is able to continue enjoying it. Issues like PSN not being available in certain countries need working out - because folks from those countries have spent money and made contracts. There needs to be some follow up guidance, hopefully we'll see it soon.