Nah, that shouldn't be difficult at all if that stack -> notebook hierarchy was implemented in an optimal way in the first place. I have a degree in computer science and had been working with SQL databases from 2010 to 2014. I don't remember any significant difference difficulty-wise in implementing 2-level vs XX-level parent-child hierarchy. The UI changes also shouldn't be all that difficult to implement. Of course, doing that will require some work, but hey, you gotta work for your money.
Now, if there are so many people have been requesting this feature for so long, there must be reasons that are valid enough and you should at least consider hearing them out and maybe even participate in a discussion, come out and tell them why this feature is not worth implementing. Maybe it breaks the core principles of Evernote mission (although I'm not sure how, lol).
Now, you're asking why am I requesting this feature. I will tell you. No, my world won't fall apart if it never gets implemented. You see, I just don't like to be dependent on tags too much because they're just not notebooks. Every note can be tagged with many tags but only can belong to one notebook, so it's a more solid structure already. Of course, you might say that it's my problem that I won't stick to just 1 tag per note. But it's kinda limiting. Maybe I want to tag a note with "Summer", "Love" and "Sad" tags but at the same time I want it to be exactly in this Hobbies -> Art -> Music -> Songwriting -> My solo stuff -> Second album hierarchy. Of course, you can always find a thousand of counter-arguments to that, like "you can do basically the same with tags, blah blah blah", but no, it's not the same, even if you can achieve similar results with tags.
I'd say just give it a shot and give your users a little bit more freedom.
Sorry, it's really late and I'm not an expert in English. I don't really care about being right and continue arguing beyond this message. I just said what I wanted to say, even if it sounded like a complete BS to you guys. I spoke from my heart
EDIT: Yeah, maybe convert those stacks to notebooks so that even the root object would be able to hold the notes in it. As well as notebooks. Basically what I'm suggesting is a folder-file approach, just like in Windows Explorer. We all know that notebooks in real life don't hold "sub-notebooks" in them but maybe make a poll and see if users would like this approach and you could get away with this concept. I'm not gonna suggest renaming notebooks to folders and files altogether as that will obviously make the brand name "Evernote" irrelevant