In that time, I've worked at 1 startup that closed a $100m C, one that closed a multi-million B, one that recently closed a $30m C, and one that started with a $8m seed.
I've started my own startup and worked with founders of other startups on the side advising on the technical side (and once in a while building the initial PoCs).
Some have failed, some have succeeded wildly, some have hit their limits of growth, some have a great product that solves an obvious problem yet get zero traction.
Here is a lesson-learned as far as "copying" goes: it doesn't matter. It doesn't matter if there are 3 companies in the same domain doing the same thing; then it simply comes down to insider connections, sales, marketing, and pricing.
In the end, the team that wins isn't always the one with the best product; there is a fair bit of luck and timing, marketing is super important, and having the right leadership team in place makes all the difference. The non-product aspects of a successful business are supremely underappreciated, especially by the technical folks. Bad products can become good products eventually; bad teams can rarely survive turbulence and it is so hard to tell if a team has the right "vibes" or not.
So it makes sense for YC or any VC to bet broadly because the reasons why a team succeeds and another fails is so intangible with so much luck and timing involved as well that making broad bets -- even if two YC-batch companies are very similar in terms of domain and product -- is just sound business.
Edit: to be clear, these are not my principles (no need to attack me personally); these are simply my observations about teams that have succeeded and teams that have floundered. I left 1 company because because in principle, I disagreed with their loose operational style in a regulated space.