You can, but the problem with using old.reddit.com is that clicking on reddit.com links (e.g. from reddit comments) will take you back to the new design. With the opt-out, you get the old design no matter what.
I think people have even go so far as to make browser extensions to automatically rewrite "reddit.com" to "old.reddit.com".
The redesign looks... well, I don't even know where to start, but it's pretty clear that users aren't the focus and they've sold out completely to corporate interests.
I'd actually be OK with some of this if they were forthright and honest about this shift in focus, but they're trying to say that the new design is 'better', etc. That's nonsense and lies.
EDIT: Oh, yeah, just remembered: IIRC the failure to link properly to 'old.reddit.com' on links that Reddit have full control over was registered as a 'bug' instead of an obvious way to drive traffic to the redesign.
EDIT#2: This is also what drove people to write browser extensions. The value of reddit has always been in the communities, not the design (or whatever).
Redirect extension to always get old.reddit.com. I've been using it for years without issue. It's one of the very few extensions I have that is enabled in Incognito mode.
But I still feel your pain whenever I'm on another computer without the extension and forget to type the old url.