Many whales wallets holding hundreds of Bitcoin have just one wallet, it's enough to hold their Bitcoin for years and here we are today, these whales still have their Bitcoin intact, having more many Bitcoin wallet either it's open source or not makes no sense.
People need other wallets because they want to spend bitcoin and for privacy reasons it is better to do that from a wallet that hodls a small amount of your total balance. For this purpose it makes sense to have a few hot wallets with only a small amount of bitcoins in them, whilst the main stash sits in your hardware wallet, safely backed up.