As to whether the possible grammar schools of the future will have a detrimental effect on the other schools in the area, I know why you say that, but I don't see how you can be so sure.
You say it because that's how it was 50 years ago, don't you? However, a lot has changed since then. e.g.
Teaching has changed as a career. It is now unusual to see a teacher who has been in the job more than 20 years, and very common to see one who has been in the job less than 5. The reason - they don't like it and leave.
Teachers are now, as a group, far less respected by the public, and by other professionals, than they were 50 or even 20 years ago.
Teachers are left wing (not all but that's where the centre of gravity lies). Was the same true in the 50s and early 60s when grammar schools followed the model that led you to say that they have a detrimental effect on the surrounding comprehensive areas.? The reason that this makes a difference is that many teachers are ideologically opposed to grammars, so its a lot less likely that grammars will eat up the good teachers leaving the weak ones to teach at the comprehensives.
The art of teaching has changed beyond all recognition. We've had 50 years of development and now there are lots (and lots and lots) of tried and tested strategies for delivering ideas and facts into the heads of young people, all broken down into different learning styles (reading/writing, visual, auditory and kinesthetic). Previously, it was 99% reading and auditory.
The jobs market has changed. Factories jobs are disappearing. There is hardly any heavy industry left. Secretarial jobs have all but disappeared. There isn't much call for middle managers any more. Schools used to prepare children to enter into the job market whereas now they seem to just focus on getting them into higher or further education.
I could go on, but you get the gist. Life is just completely different from the 1950s and we can't recreate it, even if we wanted to and we all tried very, very hard.