Google wouldn't be paying for the 3.5 GHz band
They are talking about sharing, not owning. So Google could use it, but so could AT&T, Verizon and other carriers. Heck, Samsung or Apple could have their phones use it independent of the carriers, though I doubt either has any desire to act as a carrier.
It all depends on who is willing to put up the hundreds of thousands of towers that would be required to make it work on a nationwide or even large regional basis. Who is more likely to make that investment, someone like Google who have zero towers and zero experience in this market, or carriers like AT&T and Verizon who already have the most expensive part of the investment, towers and fiber links to them, and just need to make a comparatively tiny investment in an additional antenna to send/receive at 3.5 GHz?
This "Google makes an end-run around the carriers" meme seems to be someone's wishful thinking wet dream. Someone who hasn't taken the 10 seconds required to think about the scale of investments Google would need to make versus the investment the carriers would need to make.
If Google does this, it is FAR MORE LIKELY it will be for fixed wireless internet, not mobile, and they'll do it on a small scale in a handful markets, like Google Fiber. It'll garner a lot of press but change the lives of only a fraction of 1% of people. Phones will not be able to send/receive on 3.5 GHz because Qualcomm won't add this frequency to their chipsets because it'll have so little use. Unless the carriers decide to use it, that is.
Google would offer a little antenna you can put on your house to can get wireless internet, instead of running fiber. It'll just be another way of getting internet, available to maybe 1% of the population someday. Yawn.