Building systems just so they will exist, or not
What would you build if you knew it would be practically guaranteed to succeed? Suppose you find yourself 20 years in the past with $100K in your pocket and a perfectly valid identity. You have credit cards, a social security number, a driver license for your state, and all of this. You also possess the knowledge from having already lived through that time once.
One example might be creating a dialup Internet service in 1993. You'd know that you'd get a bunch of customers and could fan out into other things like web hosting, dedicated circuits, and DSL. You could also position yourself to be purchased by one of the companies which went around eating smaller ISPs towards the end of that decade. You'd have a bunch of money and could do anything you wanted.
Or you could turn around and get into satellite TV. By the mid-90s, minidish systems would be popping up all over the place. Like so many of these ideas, it could just come down to getting there first. There are also the possibilities like heading to Vegas and betting on anything you can remember. If any of this sounds familiar, then you may have read the same book.
Somehow, these things don't seem to excite me all by themselves. They all seem like building something just because it's possible and not because it would solve any particular problem. That is, looking at those possibilities from this vantage point as a thought experiment, they all seem like "spec" projects. It's the development equivalent of "because it's there" as far as I'm concerned, although here it's "because it can be done".
I suspect this general feeling would apply to any era, including the present. Doing a project without an actual user in mind just feels fake, forced, and unrealistic. Even if you told me something like "in 2020, at least 10 different companies with technology X will be worth $100 million", it still seems to be missing something.
Let's change the scenario a bit, then. You still find yourself in the past with all of those resources, but then something different happens. You happen to run into someone who's trying to get something done, and the solution is to build something using your future knowledge. Now it's very different. There's an actual person or people who can directly benefit from your work.
It is no longer something being built just so that it will exist.
I suspect this probably does not apply to everyone. I would love to talk to someone who happily builds things just because they can. It must be a very different life, and I want to know what that's like.