Let's try to think quite ahead, going sci-fi even:
Maybe the only use case for such programs is valid if they are powerful AIs, or uploaded human minds. But for the blockchain to be able to run that? The compute and memory needed is astronomical. It also means that blockchain will be relevant technology only in the 22nd century and now we are just playing with it for no actual value whatsoever.
Because, how else would I ensure that my uploaded mind from year 2060 henceforth runs forever in the cloud? I can't, the 22nd century equivalent AWS may be shutdown. Solution - blockchain...
Quantum computing is going through this too right now where we have the ability to compute with a handful of qbits, but the implications on a years time horizon are profound.
You do bring up a valid point in that it’s unclear exactly how the “legacy software” story fits in here. Migrations to new blockchains and the blockchains themselves upgrading without contention (Tezos comes to mind with this as a core feature) seem like the natural response to this problem. Not unlike anything else though it’ll be hard to predict until it slaps us in the face lol.
There's actually something truly new and tangible that crypto has spurred - the ability to drive consensus in a trustless manner automatically and the ability to execute computer programs that are deterministic, immutable, and will run forever. With those new tools in the global toolbox I suspect that some iteration of them will be in use somewhere in our future, but no-one exactly where. Are NFT game assets the ultimate expression of crypto? No, but Friendster and HotOrNot weren't the ultimate expression of the internet either.
The ability to network 2 computers together was also new and tangible thing that the internet as a concept brought, but it wasn't before a huge amount of foundational work in the form of IETF protocols, router hardware design & production, software network stacks, and wireless communications before it really took off. Mass adoption was one of the last phases, and I still remember a day when people would call you crazy for putting your credit card anywhere near the internet.
IMO that's the phase we're at, so for most people it's just not ready yet, but it doesn't mean that there aren't a lot of people out there trying new and novel things with these tools ¯\_(ツ)_/¯.