Re: They can only do that if...
Actually, the complete, accurate statement is "you really can't trust HTTPS".
Probably true. OTOH I personally don't much care except when money is involved. And I try to do as little as possible involving money on-line. I find that face to face, paper, and/or telephones work better and are less inconvenient than online with proper security and are less scary than online without proper security.
For me, most of the time, https mostly means I can't view a constantly changing array of sites in one browser or other (I have at least six installed) because their certificates have some subtle or not so subtle flaw this week.
My guess is that most users will have no idea what Google is about with this HTTPS thing. Depending on implementation details, they will either click through any annoying error messages or will whinge until someone shows them how to switch to a different search engine.