It's trivial to detect element visibility, this just doesn't work in bigger sites.
I can and have defeated forms that tried to do all of those things very easily in the past.
Keep in mind that if you randomize across a few variations (i.e. 4-5 page layouts), that's easily discerned if you pull the page source down 20-30 times, doa complex diff, scrub out obviously random strings, and check the total unique variations you're seeing.
That may seem like a lot of work, but consider that if you don't do it all at once, but instead roll out small change after small change, the person or people using it are not weighing to cost to do everything required to bypass it compared to finding another open mail form, but the cost to bypass just the new fix you put in place. Also, they might think it's fun doing so...
And on the site dev's side, they can just choose to outsource it to a CAPTCHA (not that there aren't services to easily bypass CAPTCHAs at scale at sub-cent per CAPTCHA rates, see https://anti-captcha.com/).
Note: To forestall any assumptions, I wasn't doing any spamming or helping spamming in any way.
It's a "don't have to outrun the bear" situation, make yourself just difficult enough that some easier target gets snagged instead.
If everyone else is incorporating recaptcha, they're all running faster than you. Even with bypass services, cheap is not the same as free, especially at the scale spam runs at. I imagine a mail form that obviously doesn't incorporate a CAPTCHA is going to garner some attention. It might work for weeks or months if it's not being paid attention to, so that's probably worth them spending a few minutes looking at.
Spam doesn't scale on a small site. Say you can absolutely fill a small site with spam comments to the point that 99% of comments are spam. Very few people visit the site (it's small after all). Fewer still read the comments. Virtually none of those will click on the (usually obvious) spam links. And still fewer will buy, making you money. If you spend 2 hours customizing your spam script to circumvent anti-spam measures on a small site, you might as well flip burgers at McDonald's, you'll make significantly more money.
Spam works at scale only when you're not customizing. I'm involved with quite a few small to medium and a few larger sites (the largest getting around 4m PI/month) and though we use WP we get virtually no spam because of trivial deviations. We get an immense amount of attempts though. The little we do get is obviously manual spam: in the correct language, with content targeted to the individual page/post content (beyond "very interesting article, I wrote about the same" one-size-fits-all).
This is as primitive as it gets. I didn’t get a single spam mail in all that time.
The idea is not to outrun your competition, it is to become a special target that would demand special work to successfully get into. Bots are dumb as long as the humans behind them don’t give them a hint how to deal with your site.
And if you’re really that valuable of a target, you can step it up a notch or even switch to google’s data collecting solution.