There's no way they'll keep the old site, and I say that as a user of it.

At some point the new site will have new features incompatible with the old and they'll say it uses too much resources to keep it going.

Well, Reddit had a (mostly) good run.

If it's anything like their mobile site, they'll leave the ability to use the old site, but it will grow more and more broken as they fail to maintain it, and eventually become unusable.

It's a bad way but old.reddit.com

"I just hope they always allow users some way to switch to the old Reddit as an option"

Pretty much the only thing why I am still using Reddit via PC in addition of mobile app (third-party)