An independent artist probably isn’t going to have an employer-sponsored retirement account like a 401k or a pension, etc. like many of us with “normal” jobs have, and are counting on to help our spouses, children, or other dependants should we die before them. Allowing them to retain the rights to an artists work for after death seems to me like it would help fill that same kind of role and also provides them a little protection, since not all artists are wildly successful and may not have been able to save much or anything for retirement/funeral expenses, etc. on their own. I don’t think it needs to last their whole life, their kid could potentially live 100 years which seems excessive and against the spirit of allowing things to go into the public domain, but I think seeing them into adulthood is fair.
Edit: I’m personally contributing to a pension at my job, my wife has never worked there but she still gets to collect that pension after I die, that’s a big part of our collective retirement plans. If we had kids, I’d want to make sure those kids are being provided for out of that pension at least until they’re old enough to live on their own. I think artists would also like to have that kind of safety net for their loved ones after they die.