How deep is it here?

In what now seems like a past life I was a tour guide in the USVI for a company called Aqua Action. We ran scuba tours in what was often the clearest water you could imagine. The most common question from our customers was "How deep is it here?" because it was so clean you couldn't tell. Our standard answer was 18' no matter how deep it was. Kind of an insider's joke amongst divers. The analogy I am reaching for is regarding deep energy retrofits. A stretch, perhaps. Here's my point. A clip board walk through usually results in Air Sealing, Duct repairs and Insulation Upgrades. Gets you to 18'. A performance audit with pressure and CO tests will recommend more extensive repairs to the home. (Deep energy retrofits) Our dive tourists rarely caught on, I suspect our homeowners don't know the difference either.
If I do a walk through audit on a home and leave behind a list of suggested repairs, there is no guarantee what the homeowner will do. Money, contractor "advice" and 1/2 measures can lead to some serious trouble. Where is the liability if air sealing leads to CO contamination or moisture and mold? At what point do we get to make-up air and humidity controls and who is responsible for commissioning? At what juncture in the repair process do we cross the line between simple energy upgrades and significant impacts on Indoor Air Quality. I have been involved in enough high performance home building to know that a really tight home takes some "tuning" to get the air quality, moisture control and thermal comfort working properly. This is a lot harder to do with existing homes. I see so many leaky homes sitting on festering crawl spaces with trampled insulation and porous duct systems. Of course I want them to do "everything" but not everyone can afford to do the work. Some of these houses are truly unhealthy.
Back to the 18' analogy. Is it good enough to hit just the Big 3?(Air Seal, Ducts and Insulation) or should we be chasing every Watt and retrofitting these buildings to the point of make-up air and humidity control. The former will impact the most homes the later is the more complete solution. Somewhere in the middle is potential for some bad building science.