Using these centralized services would be even worse then putting seed words in your will, as OP mentioned.
The information/seed can be encrypted and the people who will receive the email would already have a way to decrypt the message but they would need the email to do so.
Still it is safer and more reliable to encrypt the seed words in your will then to use some website that will likely not exist in 10, let along 30-50 years.
use Multisignature Application in 1-of-2 method..
This is just as good as giving the other person your private key right now.