Why should they? Of course they can commute, just like many highly-paid professionals choose to commute too. Nobody considers a short commute a fundamental human right.
> how far out can they get pushed before it's not worth it any more?
They won't, that's the magic of a free market. Wages/salaries will go up in the city so that it's always still "worth it".
> Do you really want them all [families] moving to the east bay instead of helping to grow the city?
I mean, why not? I live in NYC. Plenty of people I know leave NYC when they want to start a family. Or at a minimum, they leave Manhattan to move out to Brooklyn or Queens.
You can see how ridiculous the argument is, by the fact that its polar opposite sounds the same: "do you really want all the families who would be helping to grow the east bay, moving into downtown SF?"
Look, there are certainly coordination, transit, etc. problems with the Bay Area. And there are income-disparity problems nationwide. But people seem to confuse those with just being afraid of change/gentrification, and that doesn't help.