i am no tech guy in the sense of understanding it on the higher scale, and i do think you are right about the cost of running it, but i thought X stopped the free API because they wanted to stop data mining and knew they can earn money by selling the access?
and to earn from all the bots :)
...then they turned something that was expensive and risky into something safer and a revenue stream.
Twitter was just an example. At that time open APIs were the hype and everybody just assumed it would scale.
My impression is that even Satoshi thought those cost were trivial.
(and I can't think of a way to make it work without open access to an API endpoint)
Judging by what our devs focused on the last years, and how they gatekeep the API, it seems to be their main concern.