I never saw the point of masto as being open. The draw of masto is to help users block corporate surveillance and corporate/user abuses, utilizing more active moderation from people with similar values and interests. It’s a governance model. Facebook has done tremendous damage to people’s mental states, there is zero reason to think they wouldn’t continue on masto and every reason to preemptively block them. Now, if you’re on masto and want all content from everywhere, you can spin up a single user instance without any blocks (time and money). So technically it’s possible, just not as easy as joining a moderated instance.