> then what is to stop them from copying your idea after you launch?
It's a very important consideration when you're dealing with the HN crowd for example. It's not that ideas are valuable; and it's not that someone won't eventually copy you. It's that a headstart can be extraordinarily valuable to ensuring you have a shot at succeeding, especially if you're competing at a considerable disadvantage on labor or capital (much less both). Every bit of headstart matters if you're in such a disadvantaged situation.
Unnecessarily assisting your competition is just a form of self-crippling, self-sabotage. It's possible to be smarter about that.
It doesn't have to just be an idea that gets ripped off, it can also be implementation. You may have worked through some of the early difficult problems and have done things in a very specific way. Maybe it took you a year or more to get to that point, to resolve lots of problems that were in the way (the basic idea may have arrived in a flash by contrast). If you're foolish about who you show that resolution to and when, you just plausibly saved your competition a lot of effort of trial & error while you're at your most vulnerable stage. Not a saving of time and effort in the idea space (the trivial part), rather, in the very hard grinding trenches work of iterating through the early stages of a product or service (not always the case of course, as some ideas are trivial to implement). Your competition can now skip a lot of that figuring-it-out work you did, courtesy of you showing them how the idea can or should be implemented. They're free-riding on your valuable labor.
The retort to that is: yeah but they can just do that later anyway. Right, and that's where having some headstart is so critical, so valuable. This is the early aspect that always gets context dropped in these HN discussions, as though it's not a thing in business.
Having traction before the VC funded clones come after you, is a lot better than not. Do not make things easier for them; you can be certain they're not going to make things easier for you.
You'll see a lot of arguments on HN that you should just blast what you're doing widely, tell everyone, shout it from the rooftops, because if you're going to lose, you're going to lose regardless, and ideas aren't valuable, and so on and so forth. That's the false setup, it's just plain wrong. It's too simplistic in claim, when the reality is very nuanced. Sure, ideas are not the valuable thing, however a headstart is very valuable. So the point is to be intelligent about that balance, about when and who you start sharing what you're working on with (if it matters in a commercial sense to have a headstart, which is not always the case; for some projects you may hope to be copied).
So for example, if early traction for your business can be gained from a base of non-tech customers that are not as capable of or interested in competing with you, then it may be far more rational to target that group first, rather than blasting what you're doing to venture capitalists + techies (who are always aggressively scouting for the next start-up thing to pursue) that can clone you far more easily and outrun you in the resource category. Having a headstart - weeks, months, years - can make all the difference as to whether you even have a slim shot at survival.
Saying always blast out what you're doing widely, or never tell anyone what you're doing - both are more likely false setups. There's probably a better, more nuanced approach that is advantageous to your particular context.