Well, but most people are saying the company shouldn't be afraid of an intern... and you are rightly pointing out that ex-employees take what they've learned and start a competitor all the time, sometimes very successfully (I do think some of your examples are imoral if you ask me, but in business, I know that what's not illegal gets a pass however repugnant)... that's why so many companies have contracts that will forbid you from doing so (illegal in some jurisdictions, but I believe it's legal in most).

If all the person leaving is doing is creating the same thing, it's unlikely to be worthwhile. You'd be competing against an entrenched competitor with a customer base and brand recognition. What makes it worthwhile is if you want to do something different that they're unwilling to do.

For Zoom, that was to market the product to random people for free or close to free because the cost to provide it had fallen enough to make it worth while.. WebEx was unwilling or unable to do so. I'm sure it was suggested many times. Probably even by the soon to be CEO of Zoom before he left to do it himself.

Sometimes the original company is worried about cannibalizing their sales, or shifting focus from their current customers, or it's just plain a case of them moving far too slowly to take advantage of the market. These are all cases where someone leaving and starting a new company to serve this demand is a good thing for consumers, regardless of whether it's good for the original company. Companies that can't respond to market needs are inefficient, and in a well functioning market suffer for that.

In a poorly functioning market, such as one with overly onerous regulatory hurdles, or litigation preventing competition, or customer lock-in, customers are given fewer choices and competition is constrained. People taking their expertise and making new companies to serve different segments of that market is a feature, not a bug or problem. It's how the market works. If repl.it is worried about a hobby project that can't scale and doesn't seem to be attempting to compete in the market, how much value is it actually providing? Threatening litigation says a lot more about their product than the competitor, IMO, and what it says is not flattering.