Why aren't they on by default? What's the downside?
The issue I had more often is random captcha's for sites I actually need to use not letting me through. (Thanks school).
My solution for this is to keep de-googled Chromium installed, and just use it when I run across these sites.
There are a few ways of looking at the captchas; the optimistic lens is to look at it as a response to people who say that it's impossible to meaningfully reduce fingerprinting. If that was true, Google wouldn't be so mad at me for flipping this setting on.
But it does make some browsing more annoying, especially if you're not technically savy enough to realize what's going on when something unexpected happens. I think it's the right decision for them to have it off by default (at least for right now).
If you have the "restore previous session" option enabled and have grown accustomed to Firefox remembering all the windows you had open before, you may find it annoying that it no longer remembers the size of your windows; it just puts them to the default size. Although now that I think of it, this might possibly be specific to the X11/Linux version, as other window systems might handle window size in such a way that it's not affected by this.
Also, if you like having websites automatically detect if your system uses a dark color scheme and adjust their CSS accordingly, that no longer works. Again, speaking from an X11/Linux perspective here.