Well… You get to keep their money forever as well, so I would say that updates are part of the value proposal at this point. If I put myself into the mind of your customer for a moment: I have just bought your product and the latest version comes with something that reads to me as “from now on, bugfixes will only be free for two years” (not saying that’s what you’re actually proposing - just how it may sound) I would likely looking into how long after purchase I can still get my money back…
Maybe consider bringing out a new product every few years (like Farming Simulator does it) or offer new features as DLC packages.