BNB is a centralized company crypto-stock.
If you believe Binance will able to keep its position and keep being a successful company in the future, then invest in it. If Binance does good, your coin will also do good. If Binance gets hacked, your funds ain't safu.
Binance is a centralized exchange. Without binance BNB has no reason to exist.
In the world of crypto, exchanges come and go very fast.
Keep some money/difersify? Yes.
Hodl for the long term? No.
To diversify is almost always a good advice, so I agree with you in that regard. But I don't think that Binance can be equated with other exchanges which "come and go very fast". Binance is a very successful one and most likely it will be doing good in the future, so holding BNB, even long term, is a good idea imo. But of course finding a couple of other good projects and investing in them too wouldn't hurt.