Can't drill fast enough to make up for the decline in existing wells.  That's a Hubbert peak, whether they call it that or not.
Bingo.  When I talk to newbies about PO, this is the point I always stress with them.  Oil is non-renewable, so at some point you can ignore all the curve drawing and production estimates and all that and simply recognize that no amount of additional drilling or extraction will increase the rate of supply.

Once you get a newbie to start breathing again they ask a lot of "but what about..." questions, and they pretty quickly get the mental model worked out.