Private schools (educating about 8% of the population) will be terrified about this news. The thought that millions of poor children could benefit from the selection process that they enjoy, for free, will not go down at all well. They will lose a lot of customers I expect.
Not necessarily. I could already send my DC to grammar schools but the £5000 per year education they would get there is not comparable to the £15000 per year education they get at private schools. Only if the state secondaries had more resources would I be more interested in them.
And as others have said above there will be a market for private schools for those who don't pass the grammar school exams and for prepping for the exams. (Kent has its fair share of private schools while Hampshire, a county with good comprehensives, has rather few private day schools already.) I agree with a pp that some grammar schools supporters might get quite a shock when they realise their DC aren't actually going to get in.
Even if Theresa May repeals the Blair legislation blocking new grammars from opening, it's far from clear that lots of new grammars would open. Between 1979 and 1997 there surely weren't new grammars anyhow? And in a county where e.g. most secondary schools are privately run academies (and no new schools are needed), I'm not sure how the process of creating grammars could work in practice - they can't all become selective, but how would it be decided which ones become the grammars? Why would parents support their local school becoming a grammar, with a 75% chance their children won't get in and will have to travel to a more distant school?