Let's say you just eliminate the requirement altogether. What percentage of non-phone number signups are going to be "thoughtful people" compared to bots?
Sure you'll get people who refuse to sign up with a real phone number, that's a given. Some percentage of those people will be "thoughtful." You'll also eliminate a huge number of bots, automated signups, etc.
It's hard to argue letting all those bots on will not decrease system quality.
