Possibly.

Or, it's now cheaper to make (tooling investments would be paid-off). If the components are custom, then Garmin can have them made as long as they like. The 500 is a case, a battery, and a chip. It's not likely any of these are "off the shelf" components.

Or Garmin expected the sales of the 510 to supplant sales of the 500 and didn't see that happening. The 510 is a lot bigger and more expensive: there may be number of people who prefer the 500.

There wasn't much of a reason to keep both the 800 and the 810. The Touring fills in for people who don't need the extra features (for more money) that the 810 has. The 810 keeps customers for whom the 1000 is too expensive.