I think the shop is not knowledgeable on the suspension and they’re just trying to brush you off. If repaired properly the air suspension should last as long as it did originally, which in this case is 14 years.
Repair parts are not proprietary to Delco. Aftermarket parts are available to repair these suspensions and odds are after 14 years it could be more than problem. Age causes dry rot in the rubber bags and the solenoid O-rings leak. Over time the air compressor weakens or fails due to wear and moisture in the air.
If you want to stick with the air suspension then I would highly recommend looking up Eddie at American Air Suspension. He not only sells air suspension components but is also an experienced hands-on guy when it comes to this stuff.
A conversion kit is cheaper, no doubt about it, but if you like the air suspension he’s the guy to talk to.
My Lincoln has air bags and I personally prefer it over the coil setups.