It is possible that an experienced person can use an online wallet and never got hacked, but that is not enough for security reasons, anyone may later fall victim of hack and the use of an online wallet for high amount of money can result to feeling of insecurity and probably the wallet may later even be hacked, hackers should not be underestimated. The probability of getting hacked while using online wallet can be negligible for experienced users but that does not mean the wallet hack is not possible.
There is no guarantee of security for online wallets because hacking attacks will always be there and will never stop, hackers already know the huge vaults that make them rich and are always looking for bugs to access wallets through backdoors, don't hold assets on online wallets and assets on exchanges will incur feeling apprehensive about being hacked, so I always update a new wallet if using an online wallet and it will automatically transfer assets to a cold wallet.
The safest are cold storage and hardware wallets, but users should safe their seed phrase safely because offline attack is also possible. But users that do not know about phishing attack can be hacked by entering their seed phrase on a site or through other phishing attempt means. Even hack on a cold storage or hardware wallet can be possible through clipboard malware in a way the online wallet used to initiate an unsigned transaction changed the address to a hacker's address.
Phishing is the biggest case associated with hacking hardware wallets or wallets from open sources, even though phishing cases are user errors not verifying the source of the wallet address and not checking applications from official sources, so every wallet user should have learned things that are prohibited from accessing and wajid knows how to protect and enhance the security of access to online wallets and hardware wallets.