Another great example of why proprietary connectors are stupid as hell. I'm going to be upset when my 2DS XL charger breaks and I can't get an easy replacement.
in this case, apple display connector (adc) predates dvi, so they didn't really have any other option for supporting an all-digital signal path to an lcd monitor, especially not one that could also power the monitor and provide usb.
this happens a lot with apple proprietary connectors. lightning predated usb-c by almost 5 years, and it was a much better connector than the existing industry standard at the time (usb micro-b). it didn't really start to feel like a problem until many years later when usb-c started offering most of the same advantages and more.
Thankfully, at least recently, most devices have settled on only a few standards. And some older devices can be modded to work with newer ports. I know rhere are some mods to add a usb-c port to the 2ds, and from what I understand they're not too difficult to do.
It's not worth the cost. There aren't enough 20+ year old macs in the wild who need to connect to a DVI monitor to make the assembly commercially viable after tooling costs.
Though it's a very good representation of why non-standard connection schemes are a terrible idea. I would say that outlawing black mock turtlenecks would be an appropriate punishment, given that Steve Jobs is dead and we can't kill him a second time. But that would also seem unfair as it would mean we would have to all see Panos Panay shirtless all the time. That just punishes the rest of us.
Alternately, you could go with an older, slower, possibly cheaper PCI video card as well. You could use a PCI video in combination with the AGP card. Not sure if the AGP card is missing, if you can run with a PCI video only.
ATI made a couple of PCI Radeon cards for Mac, 7000 and 9200, I think, with VGA & DVI outputs. Might be hard to find these days.