I see it more nuanced…
- It depends what you’re doing with the code, e.g. mainly for personal use or for sharing at large?
- It’s not all or nothing: you can choose to use some Unicode characters that give a clear readability benefit and are supported by many fonts.
The second point is actually a reason why I made this font comparison website: it lets me check quickly if a character is supported by SF Mono, Consolas, Liberation Mono… But I must say in practice it’s not a great solution (for the problem of knowing what’s safe for sharing): the default OS fonts have few characters anyway and you don’t know what the fallback font will be.
Note that most common problems are with combining characters such as \vec. I think by restricting yourself to non-combining, you can have some nice characters that are pretty safe for sharing.
There’s also the fact that improvements come with a price. If everyone stayed with ASCII because that was the only safe option, we would still have English as the only well supported language on Internet… You need early adopters willing to sacrifice some compatibility to improve the status quo.
But most importantly: for personal projects that are all about having fun (and not conquering the world), I think it’s fine for people to use whatever characters they want if they have fun that way 
