lol or what? How do you make people do a thing, anything, 20 years later?

Code escrow.

You factor in the expense of having your code releases escrowed by a third party (where part of the escrow contract itself is: "must be buildable from sources as provided"), and have a post-release pipeline that automatically uploads the new version. At the end of the term, the escrow holder releases all the versions.

This is a fairly common arrangement in high finance. If you want to supply services to a bank/insurer/etc. they will typically require an escrow arrangement as a contingency plan against you as a vendor going away. And yes, they pay the escrow costs.

There are countries that require published material to be submitted to a national archive[0]. A similar system could be done for software source code and made public on expiry.

[0] https://youtu.be/ZNVuIU6UUiM