My understanding is, this is not because Bayesian filtering solved the spam problem forever. It's because Bayesian filtering solved the spam problem for long enough for Gmail to dominate email.
Google had a ton of data and they were already using it to recommend ads, so they were well-positioned to use it to make the best spam filter. Unlike the things that came before, you could rely on Gmail's filtering. You didn't have to sort your spam folder ever. This was a clear advantage bringing even more people to Gmail.
Now the thing discouraging email spammers isn't any given filter, it's the fact that Google owns email and the cost of messing with Google is too high.
(I am not saying that this, in particular, makes Google evil. Another way to look at it is, Google saved email as a usable medium, by centralizing it.)
In fact, I use a separate (non existing) email address for every individual or business I communicate with, and their mails all end up on my administrator account.
When I receive spam, I can easily recognize who sold my email address, and bann them forever.
That's very effective.
In the other case, just create a new address.
Was this ever true? It's certainly not true now. Gmail correctly filters almost all spam I receive. And, it also incorrectly filters a healthy amount of legitimate mail, so you still have to check your spam folder. It will even filter mail from Google.
Right now, I see mail from Google in my filter. Because it's automated dumb shit where Google Photos wants me to look at my photos from a year ago, and I already told Gmail that was spam, and it believed me.
If I miss an e-mail because it was caught in my spam filter, that's the sender's problem, as opposed to pre-Gmail, when it was my problem.
It seems to me that I haven't _had_ to re-train my spam filter by rescuing important ham messages from the spam folder in a decade. Perhaps your experience is different.
Very much so. For example, earlier this year I sent email to a company inquiring about an opening. Their response went straight to spam.
In 2006, Google definitely had the best spam filter. Now, instead, it has the harshest consequences to being caught spamming.