Very interesting read, although that's probably because my cofounder and I had nearly the same exact experience you did. We got an interview for this round of Y Combinator with an idea that could be summed up as, "it's pretty much an improvement of X."

But ultimately what any pitch comes to like that, whether it's the innovative minds at YC or the cruftiest, oldest-school VC, is always the same:

a) If X has competition and it that competition already has existing traction, they will point out it will be hard for your idea to catch on because of established players.

b) If X has competition and it's bad, they will point out that nobody will adopt X to begin with even if your version is better (generally "chicken and egg" problems)

c) And if any of them have any direct experience with X, they will point out they already did this and it didn't work.

My cofounder and I ran into scenario (b) and despite spending the spending the majority of our preparation time on (what we thought were) convincing answers, we didn't get accepted either.

If I had one regret, it's that we had a potential pivot for our idea that I think could have been spun off as a completely original idea W, and we spent barely a minute talking about it. We brought it up, PG went into brainstorming mode with us, and I walked out wishing we had just pitched W to begin with. But perhaps that story is for a blog post of my own.