Honestly, the cartridge system was a (pardon the pun) game changer. Sure someone did it first but Atari really made it main stream. It allowed for a lot of what we see today. Sure you were dropping what was about a $1,000 in today's money on a toy for you and your kids but you could play so many games on it.
The idea that you had a machine in your living room that you could change what it did just by putting in a new cartridge was mind breaking in the 70's. Sure your TV could play 5 or 6 channels maybe but it still was a TV, getting a Atari made it a home entertainment system. The whole one machine that did so many different games paved the way for the whole home entertainment universe we have today.