Interesting idea, but what if the company goes bankrupt in the meantime?

Have a third party hold the keys, uspto or similar.

you would need the USPTO to hold the keys anyways to be able to effectively resolve disputes.

Sounds like a great application for a blockchain

A blockchain actually seems a bit pointless for this because there is no 'good' being spent, and likewise there is also no need for a strict ordering of patents - meaning the "chain" part opens you up to unnecessary problems. Such as, you could have a silly 51% attack where the network discards existing blocks/patents for another chain of blocks/patents, even though there are no problems with simply keeping all of them (Because no inconsistency is created by just keeping the data from both chains).

I think an easier way to solve this is to use something like torrents, and have the USPTO sign the data being sent out with a known public-private key pair (So you can verify that what you're being sent isn't just junk someone generated). These can get copied by anybody who connects to the network, verified that they are actual patents, and then when the USPTO distibutes the encryption key anyone with a copy can decrypt them. In this situation, there's no such thing as a 51% attack because there is no history to rewrite, and you can't dump fake patents into the system to attempt to 'overload' it without having the USPTO's private key. An effort could be made to delete some information by simply refusing to replicate it (And hoping others also refuse), but as long as someone has a copy there isn't anything an outside force could do to cause them to delete it if they don't want too, and if someone makes enough of an effort someone else should be able to replicate it.

As a bonus, use a purposefully weak encryption algorithm, effectively putting a cap on how long secrets can be kept.*

*This is a terrible idea

What would the inventor get in exchange for doing that?