1. "If the original purchaser sold the gun but didn't keep a record of who they sold it to their in hot water too." No, they're not, since they're not required (in most states) to do a background check or keep any records of a private party sale.
2. While the 4473 records in theory allow for tracing, the way the system is set up makes it both incredibly manual (looking at scanned paper records, in many cases), and is explicitly prohibited from being used to actually track the sources of more than one gun at a time. So, while there's no technical barrier to doing this, ATF is prohibited by law from being able to say "1% of guns sold last year were recovered at crime scenes, but 38% of the guns sold by Joe's Guns were recovered at crime scenes, we need to take a close look at Joe's Guns, since there's something going on there."
The first problem could be easily solved by removing the private sale loophole for background checks, or at least require submission of a scan of the buyer's ID, along with a photo of the buyer. Could be done in an app, would take all of two minutes to do. Zero inconvenience for legit gun owners.
The second problem could also be easily solved, by ending the practice of forcing the ATF to delete the 4473 data, allowing for it to be obtained instantly, and allowing law enforcement to use the data to determine patterns that will allow law enforcement to target the small percentage of dealers who present the biggest problem.