This is a terrible philosophy for legislating. It undermines the rule of law, i.e. that you should ex ante be able to determine if what you're doing is legal or not.
What you're describing is rule making. Congress regularly does this, in passing a law that requires such and such agency propose (or even implement) rules that achieve this or that within so many days.
We don't say murder laws are bad because there's no way to know in advance if "bashing someone's head in with a pipe who dies a month later" counts as murder.
Yes, but intent alone isn't sufficient. We need a precise, side-effect light definition of the kinds of activities we want to ban and by whom. To date, I haven't seen that.
Passing a law which bans "surveillance-based advertising" with little more specificity is a recipe for disaster.
Imagine how quickly companies would only save minimal personalized data.
That being said, there are great alternatives to GMail that you can use, both free and paid. FastMail, Proton Mail, Tutanote and Vivaldi.net mail are just a few options you might want to try out.
Gradually Google became more and more an ad company. In 2007 they bought DoubleClick that was one of the leading ad companies on the Internet. This was part of them moving towards providing ads not only on search results, but on sites as well. At the time most ads were site or context sensitive.
So Google has been making money off ads for a long time. More or less from the beginning. The big change, however, was the move to surveillance-based ads, where the ads depends not on the site or page you are viewing, but on the person viewing the ad. That is where it went all wrong.
Once banned, Google and Facebook will submit. They will attempt to lobby against the reforms, eventually saying "it will prevent legitimate business: it represents a small fraction of our revenue, but we are selflessly lobbying on their behalf to ask you to implement this technically specific law to reign us in". Ignore them. You don't listen to the hitman when they comment on homicide laws.
And ensure that the penalties amount to a ban. The US congress and courts can and do terminate human lives. Whatever penalty they propose on abstract legal entities is not too harsh; even if they completely dismantled Google and destroyed all of their economic value, it is nothing compared to the things we do to natural living breathing humans in response to criminal behavior.
Profitable companies will submit to a law that aims to control their behavior.