It's hit or miss. There are a number of photos you can find online of a Garand blown up (including the receiver ring; not just a burst case) by 1947 military ammo. You can also find folks who fired FA 1929 ball ammo and had no trouble. You are correct about how long you can keep old powder, but keep in mind a jar of powder is only as young as the oldest powder in it. So if it was blended with something that was already nearly 40 years old, then it's not going to be as long-lived in the jar as you hope.
By the way, this is a worsening situation. A lot of the old powders you have, like your H4895, was an unblended bulk powder made for the military in the U.S.A. and surplused out of stock. It was surplused-out because the military uses a very conservative 20-year stockpile limit for storing double-base propellants and a 45-year limit for single-base propellants or the cartridges loaded with them (where pull-down powder comes from). It wasn't until the ADI produced AR2206H for Hodgdon that H4895 ceased to be surplus, and that would have been sometime in the 1990s, IIRC.
Hodgdon told me a fresh lot of unblended spherical propellant should last 75 years if kept in a cool place, but they only claim a ten-year shelf-life. All the powder companies have tightened up on burn rate specs since then, so there is more blending now than there used to be. So I don't count on powders purchased new since about 2000 to necessarily last as long as some older examples I have lasted. The large number of lot# recalls since that date would tend to confirm this.
So, unfortunately, past experience with long powder life may not tell you much that's applicable to current production powder.
Norma guarantees their powder will last 10 years in proper storage. Mike at Hodgdon told me the whole industry only warrants powder will last 10 years. That's not something I'd heard before, but my guess is that means that if it goes bad after that length of time, they are not going to replace it for you.