I think the huge dangers of creating grammar schools is that comprehensives will become secondary moderns - this will be inevitable.
Why? Just don't see it myself.
Remember there are no plans to roll out grammars into every town and city. The proposal is simply getting rid of the stupid ban, so in theory, new grammars could be opened. At this time, there is no information as to criteria for determining where new grammars could be opened, if any.
There WILL be plenty of towns and cities where a grammar can be fitted alongside existing comps, faith schools and specialist schools, to broaden choice, without affecting the other schools at all as it will be insignificant compared to the thousands of kids in other local schools.
Conversely, if you're in a small town with just a couple of comps, then suddenly bringing in a new grammar could well be highly damaging to those comps - if a new school is needed, then another comp is more likely the answer.
It's all a matter of proportion.
But the same applies to selection by other criteria which is just as bad. In my town, there are two huge faith schools which are the outstanding ones, the comps don't stand a chance. Despite empty churches, it seems a majority of parents find religion just around the time of school admission applications to get into a well-performing school rather than the failing comps!
Choice is a good thing. In densely populated areas, you're always going to have more choice, whether it's shops, GP surgeries, hospitals, etc., so a wide range of different kinds of schools is fine. In small towns and villages, then you have much smaller range of choice, there may only be one hospital, fewer shops, etc so there is no scope for different kinds of school - all being comps is the optimum. That's life - yes, disadvantaging the residents, but that's all part of the pros and cons of choosing where you live.