Garmin is on top because Garmin has figured out how to make a great product that people want, and nobody else really has.
I know another reason... and it comes from a Garmin employee... and not any we both know... ha...
The GA avionics division doesn't have to make a profit. Or very little. Research and Development costs are not charged back to the products.
I can neither third party confirm or deny this of course, but it makes sense.
They make money on subscriptions and a small margin on sale of new, but it doesn't have to cover the R&D completely.
This is only light GA of course. Plenty of profit selling integrated flight decks to manufacturers of aircraft, and once you learn Garmin buttonology you're probably going to want it further up the food chain also.
A very Cisco-like approach.
All the other avionics companies, probably can't do this. Well King/Honeywell in theory could, money-wise, but as you've mentioned, they're so far behind on UI, it'd take an enormous effort to catch up. Or they'd need to do like any other tech company does and just steal the technology and plan to sell enough to pay the lawyers and the out of court settlement ten years or more hence, after it went through the court system. Garmin would ask for injunctions and if Honeywell were smart, they'd scream (falsely) "anti-trust" to the press to stop that from happening, and pretend they're just the victim being run out of business by big bad Garmin.
LOL... none of the above is going to happen, though. Honeywell sucks.
Avidyne, they had a lot of problems but they do seem to have the capacity to learn. What they don't have is a huge cash cow of other consumer products to back up whatever they feel like doing in avionics.
You won't find any P&L numbers in Garmin's public filings for light GA products broken out the 10K. I've looked.