The defensive infrastructure keeps chasing the offense, but the academic literature predicted this exact dynamic: better filters increase aggregate send volume because senders compensate for filtering losses by sending more.
There is a longer argument here about externalities and what the inbox looks like in the future which I just wrote about here if you're interested: https://docs.google.com/document/d/1jaogzeZuYetB0z9yNiSqOPTL...