arXiv is more than twenty years old at this point. I think the rising opposition to exploitative publishers has ratcheted up pressure on other institutions to switch to an open access model more than arXiv suddenly causing policy changes.
In 2019 there was a big dustup when people thought Trump was going to issue an executive order requiring all federally-funded research to be open access. ACM as an organization signed a letter with a bunch of publishers opposing this idea. It's own membership (myself included) objected pretty strongly... and the next year ACM announced plans to move to open access. So, just maybe, they're actually listening to their members.
In 2019 there was a big dustup when people thought Trump was going to issue an executive order requiring all federally-funded research to be open access. ACM as an organization signed a letter with a bunch of publishers opposing this idea. It's own membership (myself included) objected pretty strongly... and the next year ACM announced plans to move to open access. So, just maybe, they're actually listening to their members.