Have you ever put 6-18 months of your life into a project that got canned? If you haven't, I'm not sure you should make that kind of statement without having actual experience knowing what that's like.
It’s a balance. I more-or-less learned the first time that I needed to be getting more out of it than the “release success”; nowadays I’d codify a specific part of that as “always carve out the time to improve your skills as part of the project.”
I’m teaching the computer how to do something, at the same time I’m teaching myself how to do it.
That said, when that’s not possible - At least one project had at least a few weeks (it’s been awhile) of stupid wrestling with undocumented Xcode CLI internals. That was nearly completely wasted time. It sucked then, and it (dilutedly) sucks now.
A project I put years of my life into got killed off in a merger and it’s still the most demoralizing event of my career.
Careful not to imply that other's don't. I'm pretty passionate about what I build, but cancellation is a fact of life. Doesn't mean it wasn't fun to do the building. One of my projects at Microsoft was something that was always destined to actually be sold by hardware partners, and the hardware partners released devices that were far, far too expensive so it flopped and the whole thing was canned. But we made an excellent product that won some awards, and frankly, knowing we killed it on the software side is good enough for me.
Kind of a stretch, but all of my hobby projects are like this. I have spent years on a TUI library for Swift, without any intention of releasing it. I do it to understand how terminals and layout systems work. In this case I am more motivated by the knowledge gain and experimentation than actually having others use the product.
On a less cynical tone, 6-18 months on a project, then we change company, or get fired, or the company shuts down for good. Some of my startup customers pivoted or shut down. I got paid, not my problem.
Anyway, it's better to work on successful projects. They expand the business.