As I stated in another comment it is not impossible that they may leech the fediverse to death but I think its highly unlikely. The fediverse is much more than just a decentralised platform. It is an amalgamation of many platforms with different userbases and different goals. In order for the fediverse to collapse, everything would have to be replaced together as well as the flexibility to continuously integrate new services, as is the case here now.

In the case of XMPP, the community became a passive spectator of google's advance and was eventually replaced by it. But as long as the community does not become dependent on the big corpos in any way and regards their contributions more as a nice bonus, something like this will not happen. It is this self-sufficiency that allows the freedom to go one's own way and to keep the power decentralised in the community. I have to admit, however, that this can be a big challenge, but one that is nevertheless manageable.