For an org that pretends to care about privacy you'd imagine there'd be a way to avoid, you know, the biggest privacy invader on the planet.
Just allow monero payments or something. Alongside Google play for the sheep that want to use that.
This is the right solution. A one-time payment in crypto, say $5, ought to be enough to prevent spam. That being said, Signal has demonstrated (when presented a warrant) that they do not store phone numbers. If I remember correctly all they stored was an account ID and a UNIX timestamp such as the last login.
How do they perform address book matching without an phone number? It just being accessible by SGX does not really count as not storing it.
Problem with phone numbers is they are to short to store as a hash, since you can brute force the sha256 of an phone number in a trivial amount of time on a single consumer device
In terms of what they do with big data there's more evil parties like Palantir but data abuse starts with collecting it, and I would object to it even if Google promised to only use it for good. For me my privacy is already violated when my data is collected, not just when it's abused. And I do consider Google's use of that data abusive, just not in the worst ways.
I've literally registered a Signal account on one of the free SMS sites floating around. Why would spammers choose the payment route over phone numbers? They would just choose the one that's cheaper.