It looks like the 300XL design is now 20 years old. I know that in the aviation world, that's hardly old, but in the electronics world, that's an eternity. Often, with stuff that old, several of the key components can only be had through new-old-stock. The LCD display is usually a good example. These are often custom-made and will have a limited run. They are repaired by taking a display off the shelf that has probably been collecting dust for 15+ years. The cost of starting up a run of parts is extremely high. There's no way they could justify remaking the part for anything that wouldn't approach the cost of a brand-new current navigator.
Part of the problem is that for these parts, there is no in-between. For mechanical parts, custom machined one-off parts can be made for high prices when mass production methods are too expensive for the volume. But for many electronic parts, that option does not exist, especially when the certification does not allow a substitution of more modern parts.
So, I get that we're frustrated with Garmin and other manufacturers, but the fancy features come with the expiration date attached. It's part of the pact with the devil for this level of functionality. It's not forced obsolescence, where it's a cynical move on the part of the manufacturer. It's real obsolescence, where the part just cannot be made reasonably anymore.
