As far as I know, offline storage is also very good for the long term, although it also has drawbacks. Such as being more difficult to use, risk of physical loss. I think in the end it comes down to the purpose of the storage. All storage has risks. But if you have the funds, using both is very good, meaning separating the assets you have. The key is in management. Always anticipate possible problems that could occur.
Offline storage is good and recommended but it's not enough.
To be enough, it must be started with offline creation of your wallet, then you store your wallet backup offline, and best if you can store it in an air gapped device. Offline and air gapped devices are different, as with offline devices you don't remove hardware stuffs that can be used for connecting devices to Internet. With air gapped devices you already remove all those hardware things, and it's completely safe and offline after that.