A big chunk of the tech industry doesn't think in 7+ years long terms. They think in "next seed round" terms. Therefore, they want senior devs now so they build

A big chunk of the tech industry doesn't think in 7+ years long terms. They think in "next seed round" terms. Therefore, they want senior devs now so they build whatever they need to hopefully hit the user count for the next seed round. If your company thinks short term (e.g. next 2 years) like most tech startups do, hiring juniors and training is out of the question.