If you're saying that most of these failures are a result of poor ductwork not having the proper flow rate, that falls squarely on the installer shoulders for not properly assessing the home first.
If a customer is not willing to upgrade their ductwork and make the system compatible with a variable flow, I think the tech/salesman has a duty to simply not sell it to them.
Sort of like if an idiot customer insisted a tire shop mount a tire on a rim it wasn't compatible with. The shop should simply say no.
I'm not saying the customer is always right and they can't be tightwads, but something tells me most of the time it's about making the sale on the higher priced equipment and not the technician pleading with the homeowner not to buy the higher priced equipment that won't work in his home.
You obviously haven't worked for a residential HVAC company. Or a commercial HVAC company. Union or not.
The home is assessed by a so-called "Comfort Adviser" (CA), not the installing technician. The installer might give some basic info, but an office person will usually close the deal.
The CA is usually an office person and the installer is a field person, so you know who is right, and who is wrong, right from the get go.
Damn near all of the time, if I bring something up about an installation, I am shot down by whoever is in charge. Put it in, and if we get caught, we might fix it then. I couldn't tell you how many times I have heard "it's just the return, it doesn't need sealing, or turning vanes" or "no one will see it"...
The salesperson/CA at a company is paid on commission, so there is no interest in walking away from a sale for most (not all).
The installer might get a spiff on upgrades, and this stuff is tracked by the employer, so there is a conflict of interest.
Places like Sears hire so-called subcontractors, and the installer is paid a piece rate, and has to hire his/her own helper as another subcontractor. Because these installers and helpers do not make the sale or determine start times, technically they are employees, but unless someone calls the DoL and reports it, nothing changes. Other shops operate in this manner, too.
A couple of years ago, the most recognized residential company in the Chicago area had a contest where the employee that condemned the most heat exchangers got a new Ford Mustang. I have yet to run into someone with a condemned heat exchanger that has proof- either a picture or a combustion analysis report.
Another big issue is that there are no specifications for residential bids so the customer can compare apples to apples. The continuation to that train of thought is that given specifications, most do not understand them or read them. The customer skips right down to the line item with the number and compares that only.
For the record, if I can't get the minimum changes required to improve airflow for VS equipment, I do not install it. Same goes for other requirements like combustion air or chimney liners or linesets or barometric dampers. I have refused jobs.