Also, in this scenario, even if the tech industry is not actively sexist, it is being affected by the sexism of the wider society - and, if you think there is anything wrong with that (eg decreased pool of potential employees/founders/idea people), then it is quite possible to make tech-industry-specific moves that counter the extra disadvantages imposed from outside the industry, like affirmative action programs for girls that give them increased access to the tools that will allow them to forge their merit, even when you can't quite identify the process that is reducing their access in the first place. In fact I think that for a field which prides itself on solving big problems, saying 'it's not us it's the rest of society' is a cop-out, and to agree with your statement and not support affirmative action in some form is equivalent to supporting entrenched sexism.