Your initial post echoes my sentiment exactly (and it's obvious from my sig.)
Not every milsurp has collectible value just "because".
The 91/30 is the perfect example. Their sheer numbers mean that no matter how many are "bubba'd", there will always be more than enough remaining in their original configuration to hold their place in history.
There is now (and there wasn't, before) more widespread knowledge of the MN and all it's variants- and the rarer, collectible ones are unlikely to go unnoticed and modified.
And you can always place an original configuration, collectible barreled action into a pillar and receiver bedded aftermarket stock, to see what it can really do with some handloads- while retaining the ability to place it back in it's original configuration without permanently affecting it's value.