No one needs to do anything, but most families living in silicon valley are probably spending more than 5k a month. 8% is also probably too high. S&P historical returns are 7%, plus 15% tax.

People with kids. My daughter's preschool is more expensive than my college was!

Wait until your nest is empty.

Who has a mortgage without interest payments?

I think in the US the interest is deductible.

A very small portion of people in the US benefit from the mortgage interest tax deduction ever since the standard deduction was greatly increased. You have to itemize (forego the standard deduction) in order to take advantage, and only 13% of tax filers itemize on their taxes in the US.

You don't start out with a mortgage, unless have some unusual circumstances.