The short answer is, it depends on the developer.
When you buy a project, you are entitled to download every file that belongs to that project, the only exception being files that have a different price (as set by the developer). So in theory, if after 10 years the developer updates that game, you will be able to claim those updates for free.
However in the real world, a developer can’t be paid once and work for many years (especially in the indie scene), so they might implement different ways to make sure their work is funded. This can be done by either making optional DLCs for a price, or making a sequel for a game, or adding some kind of subscription for extra content.
My advice would be, when you buy a game, do it because of what it looks like now, and not what it is promised to become in the future. The only exception being games that are still under development (betas, early access), in which case users are funding its development, and are aware of the risk that the game might not become a success.
This is strictly for games here. Other types of software can come with a guarantee for updates.