I'm going to expand on my comment and turn it into an answer.
I take issue with the word abandon in your question. Using
latex3 does not mean that you have to abandon
latex2e. It is entirely possible to use them alongside each other. The differences at the document level are such that it is relatively straightforward to remember "This is a
pdflatex document" and "This is a
xelatex document". I do my lectures with
xelatex because I want to be able to use
unicode-math. I tend to write my articles with
pdflatex - see below - though for the fun of it I tried an article with
xelatex and using unicode symbols and really quite liked the readability of the source code.
So when there's functionality that I would like to make use of, then I'm happy to use
latex3. If I don't need them, then at the moment I'll probably not use them so that I keep my options open. As I see more "here's what you can do" on this site, then I find myself more and more using them to make use of what I learn here. Some is a bit silly (such as my use of
unicode-math) but others are more serious.
The one thing that I really don't take any notice of is the compatibility with journals or the arxiv. This is for two reasons:
If a journal actually accepts my article then (within reason) I'm happy to modify it to suit their bizarre style requirements (and before you ask, all journals have bizarre requirements).
Before a journal accepts my article then I'll not know what bizarre requirements I'm going to have to meet so I'd rather make my life easier here and now than try to guess what I might have to do when it is accepted.
The point of that second one is that I spend a lot of time writing an article. I'm often doing the maths as I actually write it because it is often only when I see it written down that I see whether or not my argument worked - when it's in my head then I gloss over all the technicalities. So it's not "prove result then write article" it is "prove result while writing article". This means that the writing stage is longer and more involved than if I worked the other way and so it is all the more important that the writing be easy and not get in the way. So being able to make use of great tools and stuff is more important than not annoying some future copy-editor.
So if you see an example of something on this site and you think "That would be just perfect for what I'm trying to do", don't let the fact that it uses
latex3 put you off. Just think of it as another LaTeX package that you use for some of your documents, but not all of them. It's no big deal.