I don't see it as fear mongering. What do you think of open source software like Linux, Firefox, VLC and so on? Because a similar argument is made there - we need to use these tools day to day, and the only option being to pay a corporation to use them would make us literally poorer, and we also wouldn't have the features and quality we see in Windows, Mac, Chrome etc if there weren't open source competitors.
Is lack of open source AI currently a civilizational existential threat? No. Could it be if current trends continue? Potentially. Things could get pretty dystopian if only the mega rich control tools with ultra-intelligence.
There are some open models right now, but they're mostly ones created by private enterprise that were released to the public. Creating models is more resource intensive than making open source software. So it makes sense to pool resources for the public good.