The man probably trusted his family, which is a really natural thing to do, though sometimes it turns out to be a mistake, like it clearly did in this here, as the son proved untrustworthy, unfortunately.
It is not about trusting their family. It's about carrying around $400,000 worth of bitcoin on their phone instead of storing it somewhere safe. If his family member were able to recover the coins from his mobile wallet application after his death, then it means anyone could have stolen his phone and somehow recover the coins. It is not safe to store large amount of coins in a phone wallet.
Storing coins in mobile phone is not a ideal situation but in this case, that stupid son stole the coins from his father and even if he stored them in a hardware wallet, he will be able to access them considering the way in which he was able to move them from his mobile phone.
It is time to hide digital wealth from family members.

Not sure if the son is stupid or someone else over here.... How does one steal from his own dead father that probably left the wealth for their children?