>Arial is designed with the same metrics as Helvetica
Mostly true, as per https://wiki.archlinux.org/title/Metric-compatible_fonts but Arimo is designed to have the exact same metrics as Arial, which is important for situations where one is testing a web page/app and can only avoid testing a different font if it’s guaranteed to act in the very same manner. I don’t think Apple guarantees that their Helvetica is compatible with Arial to that degree. Using Helvetica in a font stack on a web page[1] isn’t practical unless one discards that 1-2% of Linux desktop users out there, since they will often get Nimbus Sans which has a different vertical alignment.
In terms of the power users with custom CSS which replaces all instances of Arial with Apple’s Helvetica, they are probably clued enough to understand it might screw up the alignment of an element tied to a font’s exact metrics on a web page. I agree it’s unlikely, but I also remember when clients insisted on “pixel perfect” HTML/CSS renderings of their Photoshop-generated layout template.
[1] If they have a Helvetica implementation as a .woff/.woff2 web font, this doesn’t apply. I’m assuming that this is a web page which doesn’t have any downloaded fonts.
I think Arial was designed in the 1980's to match Linotype Helvetica (which Apple licensed).
Pixel perfect is tough, but a quick lookaround suggests that the two fonts should match glyph widths at least. There are other metrics to consider though, for sure!
Arial is designed with the same metrics as Helvetica because Arial is a copy of Helvetica.
Replacing Arial with Helvetica is just restoring things to the original-not-encumbered-by-licensing-decisions standard.
Some people may prefer Arial due to familiarity etc (a distaste for quirky capital Rs?). Some people may not care. But Helvetica is the original.