Old consoles like my SNES, Genesis, Neo-Geo CD, and Saturn have their best outputs in 15 KHz RGB, usually through a SCART/RGB21 cable since that was the only TV standard connector that allowed for a pure RGB output. No degredation of the signal that way. The output resolution on these older platforms is also generally not the typical 480i, but 240p, which may confuse some signal upscalers/converters/processors.
Arcade boards that run in low-res mode also output 15 KHz, and those that use medium-res mode run 24 KHz. 31 KHz (VGA) is actually considered high-res.
The problem with most VGA monitors is that they can't sync below 31 KHz. It's still RGB, yes, but if they can't go that low, then they're not of much use for retrogaming like that.
I was originally planning on an RGB SCART-to-component (Y/Pr/Pb) converter with a SCART switch before it that had RCA outputs for the audio, but upon seeing the cost of that XV29 Plus, decided that it would actually be a tad more affordable if I'd just spliced my own cables. As you say, DE-15s are common, and the various DIN connectors used for the Model 1 Genesis, Neo-Geo, and Saturn should be fairly easy to find as well. (The SNES will be trickier, though...damn proprietary A/V ports!)