I don't think language has got anything to do with it. It's more about the instincts. I'm purely talking in terms of probabilities but the odds of a C# programmer coming with an enterprise background are high. As such they bring certain instincts with them which are very useful in enterprise but not useful at all in startups. At worst, those instincts might do more damage than good.
So it's more of background/instincts things than language. C# technically has nothing which makes it a bad choice for startups.
> So the use of C# dooms my startup? I feel really out of touch having never used python or ruby.
No. It may even be an advantage for some verticals such as finance, and some business/deployment models like on-prem enterprise software, especially if being able to fit seamlessly into a customer's existing Microsoft setup would be a major selling point.
That said, if you find yourself contemplating how to scale your Active Directory and SQL Server deployment[0] to accommodate your growing user base of personalized emoji GIF makers, consider that your prior experience with your chosen platform may have led you astray somewhere.
[0] Apologies if the metaphor lacks punch, my experience with the MS stack is a decade or two out of date at this point.
Not at all. Keep in mind that YC startups are birthed from a very narrow slice of the tech world. .NET Core is very, very good tech and unless you need something super niche, there's no reason to doubt your choice. Press on.
Also keep in mind, just because .NET does not show up in the top 50 YC Tech startup list, does not mean there aren't many startups using it. I am working on one such YC company. The entire server side stack is .NET based with F# and C# being the dominant language.
So it's more of background/instincts things than language. C# technically has nothing which makes it a bad choice for startups.