You didn't read the notice? That all of your D&D books will self destruct the moment the new revision is published? It's right there in the DMG ... oh, right. I answered my own question. No one reads the DMG so of course you didn't know! Never mind.
But can you see from my angle - how it may get confusing having two very similar versions of the game being played at the same table?
Do you get a 1st level feat? The 2024 PHB that John has says he gets one; Sally who made her character with the 2014 copy came without one.
Does rolling a Natural 20 automatically succeed and give you an Inspiration? John got one, but now I have to remind Sally that she gets one.
John's character has a more meaningful background than Sally does, and his dwarf has tremorsense.
As a DM, am I using the monsters from the 2024 Monster Manual? Does that upset the balance of the encounters in Out of the Abyss?
A player has read the Bastion rules and builds their character concept around that goal. Do I have the rest of the players borrow that copy of the PHB to have the same engagement with the campaign?
Do I pick and choose which weapons have special features? Do I say "we're going to use the 2014 version of Hold Person and the 2024 version of Charm Monster - so you'll have to keep both books handy."
Does the similarity of 2014 and 2024 rules iterations cause a DM to conflate the two versions and become confused?
What about organized play and pick-up games with strangers? Will each table need to have a sheet of house rules to say which variations apply?
But yes, at my tables when 3.5 came out, 3.0 was tossed out. When Pathfinder was released, 3.5 was abandoned. This wasn't just my observations - my contacts in the publishing industry told me this happened to their products. They stopped selling when the books didn't have the +.5 added to 3rd edition.