Look at it this way - long term, even if Mason jars rust, there will be much less exposure to the metal in comparison to tinned tobacco. The glass is inert and the tobacco does not directly touch it. The jars keep a seal and they are pretty damn reliable.
Sorry for my imprecision but the
it in question was supposed to be in reference to the metal. I just worded it awkwardly and gave the incorrect impression. My thoughts are not always organized in a clear fashion and evidently I did not proofread that statement well enough. lol
My point was that the glass is inert and that the tobacco does not directly touch any rusting metal (which would take A LONG TIME for Mason jars as they are much more resistant to corrosion than tins) so they are basically much safer from rust exposure than a blend kept in its original container.
How do I know? I have many mason jars up to eight years old that are pristine while I have some tins around the same age that show extreme rust damage on the inside, sometimes even corroding through the exterior.
I'd much rather smoke tobacco surrounded by glass than tobacco surrounded by rust although it is not necessarily a loss if the seal was still intact and the wrapping paper hasn't disintegrated.
The little rectangular Germain tins are the worst culprits I have seen so far but I have also heard that tobaccos using vinegar such as Old Dark Fired or St Bruno Flake are vulnerable to rust. It would pay to jar any problematic tins like this for long term aging so you don't risk losing any excellent aged tobacco.
"And unless you're in a very dry or very humid location tins will keep for months after opening without losing too much moisture."
To my taste, that is not true. We all have our own preferences. I find moisture levels going off in a matter of weeks if not days sometimes and I am not in a dry climate. I don't smoke daily so I tend to notice a difference every time I go for another bowl if I don't have my tobacco sealed up. Even when it is sealed, I have turned to Boveda packs in cases where I find moisture/flavour has been diminishing over time. This seems to revive the tobacco in a sanitary and hassle free way that I prefer to various other rehumidification remedies. YMMV
(To be clear, I don't put Boveda packs in anything I cellar - only in jars I am smoking through in the near future and which will not be aged.)