The 5200 started off as a good console and then Marketing jumped in. Backwards compatibility was fairly easy and originally considered in the design. We were told no way, no how, and don't even make it an add on. That's why the original 4 port console required a service center install a mod to jumper in composite video and audio from the cartridge port. A few weeks prior to launch some Marketing dude (Gary Blonfield I think) called over demanding backwards compatibility and the hack was thrown together.
Ron Flint hacked the upgrade together in a day and George (last name??...it was 27 years ago) took one of the half dozen 2600 derivatives I was working on and made the adapter. We had to use what was quickly available hence the old style spring switches. This was a good year prior to the first 2600jr prototype.
The lab staff proclaimed the new product the "piggyback parasite" or the "tower of power". We always called the sticks the "jokesticks" and went so far as to write a petition to management not to release those sticks. Zdybel's wrote his name in, as did Larry Kaplan back as Software V.P. and my name was #3 down on the list
We all so hated that console, and marketing! To this day I cringe every time I pass a 5200 at the flea market. So much potential wasted.
Jerry