My employer manufactures instruments and systems used mainly for scientific research. We have had to re-engineer several of our instruments because electronic components have been discontinued or are simply impossible to find. Garmin is probably having similar issues but with the added burden of possibly having to recertify the equipment after the design change. It is not hard to imagine that paying for all that would make the repairs economically unfeasible.
Just like other old avionics (King radios anyone?) independent repair shops will probably keep these units going for a long, long time by stripping parts out of used equipment.