Seriously, _what_ are they gaining by eliminating access to third-party clients? If they want usage data, they already have all the API calls. If they want more ads, they can change the APIs to inject them.
If value in a platform comes from third parties choosing to use the service, and those third parties are free to use alternatives, then platforms should be very careful about how greedy they get in exploiting their users.
Most of the platform value actually comes from future, continued use.
OpenAI should start a clone, make it nice, and train their LLMs off of it. If discussion boards have immense future value from hosting humans interacting, clearly the cost of hosting them is worth it.