The companies don't give a fuck about CCUs.
And well, obviously if they opened a Marathon closed beta to let's say 5000 players they won't expect the same CCUs than and Arc Raider beta opened to let's say 100000 players.
In fact, companies don't look at CCUs. They look instead at metrics related to ARPU, DAU, MAU, retention, which are the ones important for GaaS. Only the server engineer looks at CCUs to know if they need to add or remove servers, rebalance players among them, etc.
Regarding betas yes, these closed alphas and betas are more to test, verify and fix multiple things while other stuff still is under development. Some time later, once they do an open beta that everybody can freely join without needed to be greenlighted by Bungie or without having preordered the game or something like that, it's where we'll really see the interest of the people in the game. And where also the player feedback will be more representative because it will be from having played a game pretty likely almost ready to release.
I assume with a more complete game the feedback will be way better. But being a GaaS pretty likely the game they'll release will be pretty much a minimum viable product, with a lot of important stuff planned to be added after launch.