I think you seriously underestimate the technical capabilities that Rigol could call upon.
Might well be, but I doubt that (but I'd be glad to be proven wrong). Just look at their products, which pretty much only excel through their low price. As far as technology goes, none of Rigol's products are anything to write home about, even their top end products are pretty basic. Also, their products are cheap because they use commodity parts in their designs, which keeps costs low.
This raises a few questions, like where all the supposedly advanced technical capabilities within Rigol are showing. Or why a manufacturer that has built a very successful business model on building cheap bottom-of-the-barrel and entry-level T&M and lab gear using commodity parts would invest money in acquiring the technical expertise required to roll their own complex ASIC designs, especially when it's not beneficial to their market segment?
It might well be that Rigol wants to get away from being a producer of cheap kit to a reputable brand that is considered not just by hobbyists, but then the first steps certainly would be improving their current offerings, especially in terms of firmware maturity and support.
So no, I don't believe I underestimate Rigol's capabilities. But as I said I'm open to be proven wrong.
Converters running at 20Gsps or more are a serious systems design exercise. They end up being mostly digital parts, trying to handle the storm of digital data in real time. Some real ingenuity is needed to come up with architectures that stop the complexity getting out of control. Once people are funded to sit down and work through things like that, you might be surprised at the progress which can be made. It all comes down to finance. Keysight have fast converters not because there is something special about being Keysight, but because they had a sufficiently high value niche to justify the investment.
While of course I agree with what you say about the complexity of 20+GSa/s converters, I don't with your last statement about the niche. In fact, one of the (not T&M related) markets I work in is pretty much screaming for high speed ADC hybrids, the faster the better. The margins in this market are a lot bigger than say for high end scopes. Still, there are very few vendors who can offer fast ADCs, because as you stated correctly the complexity is pretty high which limits the number of manufacturers that can deliver them to a handful of high tech companies (plus 20Gsa/s is the lower end of the spectrum, most of the time we're talking in excess of 80GSa/s). Keysight has a ton of IP and patents on their designs, as has Tek, R&S and LeCroy, which is because all invest a lot of money into R&D.
Keysight has over 9k employees and over $2bn revenue, Tek has still some 4k+ employees and over $1.5bn revenue, R&S has almost 10k employees and some $2bn revenue, and LeCroy has approx 450 employees and some $200M revenue. I find it pretty hard to believe that Rigol (for which I couldn't find any revenue numbers, but I'm sure they are well below $200M) which has some 400 employees for their whole product portfolio (which includes scopes, spectrum analyzers, PSUs, DVMs, RF and frequency generators plus their lab gear) will have the same technical ressources as say LeCroy who's 500+ employees are pretty much dedicated to upper midrange/high-end scopes only. The number and type of patents Rigol is holding seem to confirm that.
But as I said I'd be glad to be proven wrong.