I tend to agree with the OP that only very seldom can one truly know that the all matching numbers gun is as it came from the factory. The only gun I have that I'm certain is unfired and all matching is a Mosin M44 remake. In 2021 a small production run was done in Russia, 5 of which came to Canada through Westrifle - I got one of those. There we a few Polish M44's that came in about 10 years ago, they were in original grease, all matching and appeared unfired so likely that one's OK too. BUT - other than as a collector piece, what really matters is the condition of the gun. Good bore, nice tight action, compliant headspace, no rust under the wood - then matching is a bonus is the gun was bought to shoot. You're going to add dings and scratches when you take it out, you're going to put normal wear into the barrel and mechanical parts. Buy what speaks to you, be safe and make sure its OK to shoot then enjoy it.
The reason both you and OP are of absolutely incorrect option (on what new collectors should be focusing on) is because you don't read books on subject. Including expensive books. Including books in foreign languages. Including in those languages you don't speak but sit and translate word by word trying to understand how many Roth-Steyrs had this very specific marking in this very specific place. Not reading books is not that bad, I don't read books on the subjects I don't care either. But what I also don't do - I don't advise about those subjects.
- Your 2021 M44 was not made in 2021, at least in colloquial sense of "being made" - it is a combination of refurbished old parts (some of them dated to Imperial period of MN), though I don't exclude the possibility that stock might be newly made.
- Polish M44s are not "likely OK", most of them are indeed true factory matching rifles in original unissued condition. This is a fact, not opinion.
- OP is venting out his frustration about "matching K98k" simply because he can't grasp the magnitude of information on the subject. It is advanced topic and though I have a bible from Karem/Steves I still don't know enough to be able to rely on my knowledge. But I have good collector friends who I trust in this subject and they trust me in my subjects. And OP posted the same nonsense in Red rifles forum, clearly he is frustrated.
- For many types of collectible firearms you can tell factory serial number from repair serial numbers from refurbishing serials numbers. Like "TRULY tell". But again, you need to read books. Dig into subject. And just because you and OP can't - it doesn't mean all other are not capable of this. Many ppl can, it's time consuming but rewarding hobby.
- If for you shootability of firearm is THE ONLY criteria of firearm and you're preaching it - then you're Fudd. There are hunters Fudds (who needs AR15?), there are farmers Fudds (who needs anything apart of my utility rifle?), there competitors Fudds (who needs old junk and hunting rifles?), recreational shooters Fudds (who needs what I don't understand or can't afford?), LE Fudds (who needs guns apart of LE) and collector Fudds (who needs modern guns, just let us collect our history artefacts). All of them are Fudds, pick your category. I compete and I collect, however I recognize that farmer or hunter or recreational shooter might and probably will have criteria for firearm selection that are very different from mine. And I respect this. I don't preach them with "here's my wise and insightful advice to you: don't buy mismatched SKS, buy Siberian or WK180 with ten clips, trust me, I have a TikTok channel and have an unboxing video and I fired 20 bullits, the most reliable rifles in the history of mankind".
As a summary, a couple of tips for new milsurplus collectors: keep your mind open, educate yourself, find and do what you like the most just don't be a Fudd, don't listed to Fudds, have respect for other gun owners even if your don't understand their needs and interests in particular guns.