I would guess that he was mostly pissed that things "just changed" without being asked. While these sort of things like the up arrow seem like a no-brainer to many here, showing up for work on Monday and having everything just be different sucks. It's a bad experience, and I can't really blame him for being annoyed.
A lot of times people would be a lot more receptive if you show them "hey, you can use the up arrow like so if you want!" Then they can choose and adapt at their own pace if they want to, and they're in control of their own systems.
I told him (this guy was the chair of the informatics department at a major biomedical company) that I understood if he didn't like color ls, because it could be hard to see some colors on a terminal, but that if he wanted to disable arrow key history he was just crazy. Some things are just progress.
I've been trying to stick with bash for at least 20 years (I did have a brief tcsh phase before that) since a time when sticking with it meant having to install it on every unix system I touched. However, now that MacOS defaults to zsh, I'm starting to wonder if I should summon up the effort to change again.
I think the most noticeable difference is that `$foo` doesn't word-split unless you do `${=foo}`. Beyond that, I don't think one would notice they were using zsh instead of bash. In most ways, zsh is pretty much a superset of bash.
personally, I don't have interest in zsh because bash is the default shell in Linux, which is the UNIX system I use (I find mac's unix to be really a weird variant). Mac switching to zsh is just more evidence they're diverting from the mainstream.
I know what the reason is. I don't care. I use GPLv3 and have no interest in an OS that doesn't. You can run bash on windows or compile it yourself on any platform. microsoft even distributes it with windows.
IIRC, MacOS had to ship with a fairly old version of bash by default because newer versions have a more copyleft license; I always assumed that their swap to zsh was because they wanted something both up to date and compatible in terms of the license (and the fact that most bash "just works" when you run it in zsh is a plus).
biomedical research, but the person I was helping "had written a kernel disk driver for VAX BSD back in the 80s"
and I think they wanted things to more or less stay the same as BSD in the 80s. Which, as far as wishes go, isn't terrible.
A lot of times people would be a lot more receptive if you show them "hey, you can use the up arrow like so if you want!" Then they can choose and adapt at their own pace if they want to, and they're in control of their own systems.