Sink cost is almost always a fallacy. Not for the end user, but for the admin.
The amount of work required from the admin to start a successful forum is about the same either way in terms of producing content, but fewer of the free users tend to put it in because they haven’t paid the sunk cost to get started. The paid admins realise they’ve just sunk best part of $200 or more to get started and feel a need to make that investment show something in a way the free folks don’t.
This is in no way perfect as an observation - exceptions absolutely exist on both sides - but it was always the case that those putting in the money would feel the need to invest their time too to not waste the value of their investment, even if the net total of effort worked out about the same in the long run.
This will now be the part where you try to prove the exception to the rule because you’re a contrarian. I’m just relating what I’ve seen in 16 years of forum admin observations, and more years of forum participation.