Our product is used by 2 million people and some of them are literally curing cancer and hunger. That's actually a great motivational reason for us to continuously improve it.
Yes, it's a great feeling and strong motivation to work on something like that, and it gives you the will to stick to it for the long run.
I've been working on a system for improving health and preventing diabetes since 2005 [1], which is complicated by the fact that it's designed from the start to support controlled clinical trials, followup questionnaires, and analytics to measure how well it works. That's enabled us measure how well it works (which literally involved drawing blood), publish papers proving its effectiveness, and feed that learning back into many changes to improve the system over the years.
Parts of the system really needed to be rewritten, but were originally intertwined all throughout, and there was no time for a rewrite. So over the years I've been incrementally refactoring and isolating those parts to make it easier to remove them in the future, even when we were under crunch and there was no time to rewrite.
I finally found the time to completely replace the bad parts, and it turned out to be much easier (and more satisfying) that I expected, significantly reduced the complexity and lines of code, and made it possible for us to hire contractors to work on the code without their heads exploding. But I think it went so well because I'd been thinking about it and chipping away at it for years.
