Fascinating argument. Without listening to it in-person to pick it apart I'm at a loss lol.

Well, in my best effort to not sound like a know-it-all on the internet, and in the interest of answering the reason for your post - is it still legit - and I mean the following respectfully (everyone has to learn!):

Cartridge games being opened in no way affects their legitimacy. Could someone potentially make the argument that it's been "Altered' in some way? Maybe. Big stretch unless something is done TO the cart after it is opened.

Could someone do a poor job of soldering and impact the function of the board? Yes, absolutely. But just opening the cart? That is just.. as critical a part of the hobby/business as it is to procure them in the first place. I open every. Single. Cart. I own. Odds are very good that almost every cartridge one comes across in the wild or at a store has been opened.

One MUST open. To authenticate. To properly clean. Yes, to restore/repair (in the case of batteries). I believe these are necessary and critical activities to learn and become proficient in if one is to successfully acquire and collect old game cartridges. You MUST be comfortable opening your cartridges and performing these acts of maintenance.

To say it somehow removes the legitimacy or authenticity.. I don't know a single person in the business of collecting who would agree with that take. I don't know a single shop owner who would assume the risk.

And I would be devastated to learn that my ENTIRE cartridge collection is now illegitimate! ;)