That would really depend on what kind of blockchain you design in the first place. Again, blockchain is more of a methodology than a technology. It's being used in different ways. Like I said, there are different platforms and other technologies, and they're still evolving. Right now, I don't see too many reasons why blockchain would fail completely long term. There are certain limitations. Blockchain is not a solution for everything, though a lot of people think it is. I believe a system that's well designed and planned and takes the time to get there as opposed to rushing to market will actually prove to be quite useful in blockchain.
I completely appreciate your understanding of technology and your history in it and you're right that no system is fail-safe. There will be blockchains that will be hacked in the future for sure, especially now that we have quantum computing and other advances like that coming through, but right now it's the absolute best solution we have. There will be vulnerabilities in the future for sure, but people are working on that, too.
I think that's the best answer I can give you right now.