It seems like that would only be the case with a "deterministic wallet", which determines all of your addresses FOREVER at the time of wallet creation. I believe that Armory is a deterministic wallet.
Meanwhile, with Bitcoin Core, it appears like each backup is only valid for the next 100 addresses.
You are correct. That is simply because all the addresses are created from a single seed and so when the process is repeated it creates replicable results. Currently, both Electrum and Armory offer the option of deterministic wallets.
If you want to ensure you don't lose any coins, backing up every 100 transactions is perfectly fine. Personally, I'm not inclined to download the whole blockchain (anymore) and have to consistently back up my wallet, so I'd really just advise Electrum. One solid wallet backup is all you need to do (have multiple copies spread over of course) and you don't need to waste time updating your blockchain data.