Google is way ahead of you, they are a certificate authority now, so in theory they can do this right now. Take a look at any site’s https certificate and a significant portion of them are now signed by Google Trust Services LLC thanks to Cloudflare using them to generate free https certificates (in addition to letsencrypt). Note that they won’t ever pull this trick though because it’ll irreversibly damage their reputation.

The only reason they would do this is to grab users on other browsers,

Yes that would be the purpose.

but that would mean everyone else stopping to use Google DNS, which means less data to collect or sell.

I agree, which is why I also agree with you why they haven’t done it yet, but I was speaking to how they could do it, not the fallout from them doing so.