I actually put together a service that is focused on this issue called Fidelius Vaults (https://www.fideliusvaults.com). If you have a moment to look, I'd be curious to hear your feedback on whether it solves the problem you stated.
This is exactly something I intend to add to my document storage platform... I have some notes about it here: https://www.fideliusvaults.com/roadmap/. There's a balance of security and convenience that I've been calling the 'paranoia scale', and this sits on the far end towards 'least convenient, most secure'.
While there are other document storage services, I'm not aware of any that make commitments for storing data 100+ years. For Fidelius I'm offering a 10 year notice / storage if the service ever sunsets, and I've seen similar from other companies.
The obvious concerns here are the storage media used, the DR plans for the service, and more importantly, trusting that the company will do right by you after you're gone – even if it goes under new management or bankrupts. While I always struggle to find applications for blockchain, perhaps this is a scenario where it could be useful... perhaps you could upload the encrypted data to the chain, and form a contract where the key to decrypt it is released after X years?
This is great! As a shameless plug, check out Fidelius Vaults (https://www.fideliusvaults.com): you can create an 'If I Go Missing' Folder, which can be accessed by people you trust in emergencies.
The idea is that you choose people who act as proxies in an emergency event; if something happens, a configured number of them have to approve access to your vault of documents/information before it can be viewed