Most ordinary users never make a setting change,having it off by default, would make it so those users will never be aware the features are there, and having translation available on the address bar is a major feature.
Since they do no harm, I can honestly not see a reason why it should be opt in?
The people that want to turn it off are just contrarian, and possibly oppose AI as some sort of principle, disregarding that in this case it actually helps people that want to read articles in other languages, and blind people to have meaningful captions on pictures that aren't properly captioned for the blind.

You do you, but this is not you doing you, you are being contrary at far greater cost to other people, regarding something that shouldn't even be an inconvenience to you. You just choose to be difficult without having a real reason. Except you hate the word AI.

What AI feature is it exactly, that you think is detrimental to your normal use of Firefox?

I mean heck I would prefer to not even download the components

Why? And how many other features would you prefer not to download? The plugins interface maybe? Video and audio Codecs? Support for PNG? The theming capability? How many plugins do you have? What about AI features is it that is so especially horrible to you?