Also, many, many firearms that Americans possess don't have a 4473 history...many firearms.
The FFL dealer must keep the Form 4473 on file for at least 20 years on his business premise, and is required to surrender the log book to the ATF upon retirement from the firearms business. These forms are given the same status as a tax return under the Privacy Act of 1974 and cannot be disclosed by the government to private parties or other government officials except in accordance with the Privacy Act. Individual dealers possessing a copy of the form are not subject to the Privacy Act's restrictions on disclosure. Dealers are required to maintain completed forms for 20 years in the case of completed sales, and for 5 years where the sale was disapproved as a result of the NICS check. 4473 paper forms were collected from FFL dealers by BATFE agents and stored in federal custody. Some reports suggest they are kept in Washington D.C. Other reports say they are kept in West Virginia. Of course with the debut of eForm 4473, they could be anywhere in the "cloud".
Also, many, many firearms that Americans possess don't have a 4473 history...many firearms.
A few additional points ...
1. Form 4473 is the crux of the entire gun control issue. It identifies the buyer/owner and provides the description of the firearm.
2. That is why the DemonRats have been pushing for universal background checks, i.e., private transfers (sale/loan/rent/gift/inheritance done between private parties and not through FFL dealers) must be coordinated through a FFL dealer so the FFL can create a Form 4473 and submit the transfer request through the FBI's NICS. Your point is well made that the vast majority of Form 4473s are out-of-date, i.e., the original owner has privately transferred the firearm to someone else not on record.
3. A national firearm registry does not have to be 100% or even 50% up-to-date with its 4473s to be effective. If the day comes for confiscation Fedzilla will quickly identify newer semi-auto rifle owners and go after them.
4. In 2003, Congress passed The Government Paperwork Elimination Act. BATFE responded by providing the digital eForm 4473. No new paper form record books to maintain by the FFL dealers or Fedzilla. Of course that would make it easier for Fedzilla to create a clandestine national firearm registry. Rumors abound that that is exactly what happened illegally during POTUS #44 BHO's watch of 2009-2016.
5. The chink in the Biden/Harris plan to utilize the NFA (1934) to reclassify semi-auto rifles as weapons of war and therefore get them on that already created registry is the same chink that CT and NY discovered in 2014 - firearm owners did not voluntarily comply to register their assault-style rifles and there was no way for the states to identify them.