LaVolcan, it is flawed to only consider the benefit to those not selected without considering the benefit to those selected. You also need to balance advantages and disadvantages.
On balance selective system is better suited to meet the needs of broader spectrum of ability and improve achievement for larger number of students.
It helps the most able students
It helps middle ability students in good schools
In theory it offers space to better serve those at lower end of ability, provided they were given good schools with adequate resources and methods.
The fact that lower ability students do less well is born out entirely out of political choices made in resource allocation (i.e. leaving less able behind).
This was a political choice nothing to do with selection as such.
The main barrier is that there are bad schools and that the attainment by the least able is not vewed as a priority. Selection has no incidence on this.
There is no case to be made to drag bright students in bad dysfunctional schools. There is no moral argument in that.
We need more good schools.
To help middle and especially low ability students to achieve, you need to improve outcomes in primary education and review the whole system to better serve the least able and socially disadvantaged.
Removing selection will do nothing to help standards and raise attainment among low achievers. It will only limit the opportunities for the brighter students.
On balance selection is doing good for one group and no harm to any other group.
Then there will always be those , who argue it's not fair that bright kids are bright. True, but no action necessary on this.