There will always be those who'll reduce a collectible gun, surplus or commercial, to scrap metal value in tinkering. Nature of the game.
All property is transitory, also nature of the game.
Several of my commercially-made guns of all types have been altered to at least some degree (some majorly) from their original factory configurations.
Has that affected collector value or status somewhere & somewhen down the road? Sure.
In some cases, it's lowered any projected value to strict collectors that might attach if left original.
In others, it's enhanced value, because it's also enhanced "usability", which I'm more concerned with.
You'll never see either of my sportified Mosins in a pawn shop while I'm alive. After I'm gone, or possibly not too long before I leave, both will be passed down to nephews.
While I'm here, and while they're here, they were modified to fit my preferences, and my enjoyment.
Neither was collectible.
Neither was a sin.
Even if both end up being sold sometime in the future by those nephews, I still enjoyed them, and I still improved them, as SHOOTERS, for ME.
I pay the money, I get to do things like that with what it buys.
I generally dislike the idea of chopping up a good sample of a classic military rifle, but quite frankly, with millions out there, so many of the Mosins on the gunshop rack are beat-to-hellers with warped wood, cracked stocks, wandering handguards, loose barrel bands, flaking varnish, mis-matched numbers, worn rifling, variable-sized bores, rusted external barrel surfaces, and so on (not to mention poor sight elevation regulation), that I modify my views on that as guiltlessly as I modify the guns themselves.
Neither of mine was a 39 Finn, neither was anything special, both had issues.
The Mosins you'll see far more of in the pawnshops will be the regular military-trim rifles, once the cheap surplus ammo runs out, removing 50% of the reason so many people are buying them.
The vast majority are not acquiring Mosins as collector pieces, it's very simply a matter of Cheap Gun/Cheap Ammo. Eliminate half of that equation, and the rifles will lose much of their appeal for the biggest market share currently picking them up.
It's people like me, who put money & effort into personalizing an otherwise dime-a-dozen Mosin, who are probably more likely to hang onto the things long after the surplus ammo runs out, because we have more invested in them than just "cheap". We're more likely to have squirreled away several thousand rounds in sardine cans while it's still available, we're more likely to invest in reloading dies & components to keep the guns going after that's gone.
The guy who buys a cheap Mosin & just whacks off most of the front wood to lighten it up some, is a different class, as is the strict collector who only buys the most minty examples, the widest range of serials, certain years, certain styles, and certain makers or nationalities.
There's still room for all of us.
A beat-to-spit Mosin is no holy object to be enshrined and worshipped behind glass.
Those of us who take a beater & make it into a better shooter have nothing to apologize for.
As for the true collector pieces, there's a special place reserved in California for those who destroy the originality of a Finn in good shape, right next door to the section reserved for those who re-finish an 1886 Colt Peacemaker & add imitation pearl grip panels.
For every butchered Mosin, there are millions left un-butchered.
For every re-finished 1886 Peacemaker, the un-finished originals greatly out-number them, too.
The Mosin is a durable, reliable, and reasonably accurate surplus rifle.
It is neither a refined design, nor a beautifully finished sample of the riflemaker's art.
As such, there's nothing whatever wrong with taking a $99 conglomeration of raw material and turning it into a better-looking, better functioning, and better-shooting tool if that meets the needs, desires, or preferences, of its owner.
The PU sniper Mosin, incidentally, will remain as it came.
Good shape overall, shoots OK as it sits, and that one IS a collector piece.
Denis