Arguing about "RAND(1,100)..." is beside the point. First of all, adding two-digit numbers is _not_ something that's trivially easy for a lot of humans. But more importantly, it's not leveraging any level of textual natural language understanding. It's true that any spammer that decides to target your question will be able to write a rule for their rulebase that will defeat your captcha, but the whole thing that makes the spammer's task economical is that they spend zero or a very tiny amount of person-time per spammed site. This throws a spanner in the works.

I'm also not sure I'd say that the "conventional" variety has "stood the test of time". I still see sites using them, but many of them are now so hard for humans to make out that you have to make multiple tries. And that's if you're a human with good eyesight and full cognition. The audio captchas out there are loud and obnoxious and mostly incomprehensible.