I want to say that my reddit usage will go way down, but I know that's probably not the case.
On the other hand, I know a few of the old-timers at reddit still use old.reddit, so that might help keep it alive.
In theory reddit can rebuild as an alternate front end, or someone else could do it. A good chuck of it is open source. You'd have to do a lot of work integrating the new APIs though.
Reddit's new GQL API uses different oauth tokens than the old API uses, and Reddit doesn't issue those tokens to third party apps. Until that changes, you can't build an alternate front end that has feature (or should I say misfeature) parity with the new site.
It's a shame that the most important chunk of Reddit isn't open source any more. I recently found bugs in both the old API (introduced in 2011, detailed at [1]) and in PRAW regarding bulk flair updates. Since PRAW is open source, I submitted a pull request [1], and their bug was promptly fixed. On the other hand, Reddit is no longer open source, so instead of submitting a pull request like in the old days, I'll have to make a submission to /r/bugs or message the current admins and pray that an engineer sees.
On the other hand, I know a few of the old-timers at reddit still use old.reddit, so that might help keep it alive.
In theory reddit can rebuild as an alternate front end, or someone else could do it. A good chuck of it is open source. You'd have to do a lot of work integrating the new APIs though.