You're here; therefore, we can assume you're a high-value consumer to advertise to. When enough people take the $1/month option, advertisers will be primarily advertising to consumers with low purchasing power. As that happens, the cost of advertising on Google AdWords nosedives, and Google is left making substantially less money.
Maybe most people aren't like you and I. But I don't think anyone at Alphabet is willing to risk adding complexity to potentially earn less.
This ignores the potential fallout as the message can be distorted from "Optional payment for Google Maps" to "Google starts collecting money for Maps", and finally to "Google Maps costs $X".
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