There are ways to circumvent that of course, but I just can't be bothered.
Oh mobile, consider Apollo.
1) no "this subreddit is only available via the app" walls
2) subreddit search functionality that doesn't require jumping through hoops to execute
3) no comment reading limitations (again a push for the app)
I'm sure there's more, but these are the things that pop to mind instantly
TBH I struggle to see how the new interface could be considered better in any respect worth considering. The only good feature is, I believe, showing images by default, which you can achieve trivially with Reddit Enhancement Suite and would have not required a complete re-engineering on a SPA model.
There might well be server-side improvements that make it worth to Reddit, but from a user perspective the new UI was a straight UX downgrade; without old.reddit.com they would probably have become another Digg.
I think the new interface integrates more seamlessly with ads, so from reddit's perspective, I can see why they'd prefer it.
At NetApp, backward compatibility and ability to rollback across multiple releases was a hard requirement. This made the codebase real hard to understand and maintain.
As much as many people hate UI/UX changes they happen for a reason. Due to the much more mainstream adoption of reddit I'd say that reason was wider adoption.