The(/a) rationale for UTF-16 is that it's more space-efficient for languages that use high code points, like East Asian languages.
It wouldn't be so bad if people just understood that there are (almost) no valid uses-cases for measuring the size of a string. As someone else mentioned, it's a binary blob for all intents and purposes, and if processing of its content needs to be done (such as displaying it on the screen, or performing, say, word-wrap) then it should be handed over to libraries that have been designed for this purpose, because these things are very complicated.
Simply by having a single non-ASCII character in my name, I'm seeing software fail on this on a regular basis.
The actual reason utf16 is used is because it was easier to port to utf16 from ucs16. Much the way it's often acceptable to assume utf8 is ASCII it's often acceptable to assume utf16 is utf8. Even today, I imagine there are a lot of applications which assume utf16 is actually ucs16.
Here's Dazai Osamu's Hashire Melos: https://www.aozora.gr.jp/cards/000035/files/1567_14913.html. Despite having an unusually large amount of markup (most Japanese text wouldn't add the ruby characters), it's smaller as UTF-16 than UTF-8.
But I will say that the concern you have does not take into account the concerns other people have who make this argument.