Text messages are largely free, and Google has been operating as a carrier through Google Voice for years.

I don't think they're deprecating SMS because of cost issues, I think they just want to move forward with richer notifications and don't want to spend any time working with a legacy system.

They're definitely not free for businesses, especially worldwide. Thus why twilio is a business. Expect to pay maybe $0.03 a message. I'm not sure how that compares to advertising revenue..

So why aren't they shuttering Google Voice, where people use up millions of hours of speech and SMS every day?

I'm sure Calendar SMSes pale in comparison.