I had a similar experience with my car; the shop spent quite a lot of time diagnosing my trouble. They checked parts, swapped in new and used parts but in the end, they gave the car back to me as it was. I was charged for some diagnosis time, but they forgave a part of the bill. I was happy to pay the bill and wouldn't have a problem paying the full bill. The shop, like yours, could have told me that everything needed replacing. Instead, they said that the O2 and air flow sensors look bad as do the throttle cutoff and body switches but that the trouble was mostly liveable. (The car has since worsened, which is another story.) I suppose the shop expects to get the job to replace these big dollar items so a little goodwill on my diagnosis is just good business.
I don't like to spend money, but it seems to me that spending hundreds on diagnosis time has saved you and me some big money. That is good value to me.