I'd like to think so, and I hope I'm wrong here, but probably not.
The decentralized communities are definitely going to grow over the coming weeks to years, and that's awesome and very welcome, but I don't think they'll ever reach enough critical mass to supplant the big players.
For example, a lot of people I know are fully aware Facebook sucks, but it's also the only way they are able to keep in touch with certain groups of people (family, old school friends, etc). There would need to be some coordinated effort on all of their contacts' parts to move somewhere else, and a lot of them fear losing touch with those that can't/won't make the switch. That's also not factoring in the learning curve to leave a familiar platform. I'm thinking specifically of grandparents who are on FB, but it doesn't apply just to those.
I do think there will be a steady influx of users from the big players and the content quantity/quality on Lemmy et al will definitely improve, but I feel the decentralized platforms will always remain secondary to their "big tech" counterparts.
There's also currently too many steps for users beyond "install app, turn off brain, and start scrolling". As sad as it is, that's all many people want: an app that lets them mindlessly scroll.
And, IMO, that's not necessarily a bad thing. I feel like Reddit has experienced a lot of "brain drain" the last month and more quality content is appearing on Lemmy rather than Reddit. It's been absolutely refreshing not seeing screenshots and reposts from other social media in my feed, and I think the barriers to entry for decentralized social media are contributing to that.