Well I do see the advantages of what your suggesting, no depute there. Searching for a specific tag would make my life easier but at what cost?
As I was saying a person - not a company - won’t likely be receiving that much important letters to the point you can’t simply go through a couple of folders and find out what you’re looking for. Paperless-ngx could indeed make me save a few minutes while searching for documents but then, what about the amount of time and effort it would be spending keeping the software running, up to date, backups etc? More importantly, what about longevity? Those kinds of archives are something you may want to get into 10 or 20 years to look for a file and then software you chose might not be around or working anymore.
The extra minutes wasted while searching and having the piece of mind provided by simple folders and PDF files seem to be a good tradeoff as it eliminates the need for databases, upgrades, special servers, formats and whatnot.