As of .net 3.5, the .net framework contained 98 Assemblies, 309 namespaces, and 11,417 types. The framework is gigantic, and after you've spent long enough in .net, your domain specific knowledge of the framework itself is gigantic as well.
As someone who has been building applications on .net since beta, I have no fear at all in learning a new language (bring it on!). I have a fear of losing over a decade of experience with the thousands of classes provided in the .net framework that I use, and rely on, on a daily basis. I know exactly what I'm looking for, how it works, and what the performance ramifications are of each component I choose to use. I know what needs to be replaced with something custom to scale. I know what's doing extra work I don't need. I know the ASP.net page and control life-cycle (intimately), how the view engine works (and how to add features to it), how to replace critical components for custom functionality and how to build custom controls. I know the WPF layout system. I know the quirks in windows forms and GDI+. I know programmatic drawing in both frameworks. I have done machine learning in .net. I have done massive file IO in .net. I know the .net multi-threading model. I know how to do code generation in .net. I know how to use raw sockets in .net. I know how to bend this massive framework to my will, and get all of the performance and power I want out of it. I have built reliable, predictable, massive scale applications in .net. Over my entire career I’ve invited product managers to give me their crazy ideas, and I have never had to push back.
Having said that, I also don't want to learn Ruby. I don't think it's mature enough to get my attention. I don't see how it is in any way better (that's not to say that it isn't, I just don't see it). Is it easier to learn? Maybe, but I've already crossed that hurdle; just like OP on reddit has. Is it a faster platform to blast out a simple website? Likely not for me. Is it a better platform for building and scaling massive applications? I doubt it.
Your language/framework defines the outer capabilities of your application, the performance of your application (read: scale), as well as what parts of your application can be reused from parts made available by others. They are things to be mastered. .Net takes a long time to master. The "jack of all trades master of none" attitude is what pains me. Show me that ruby is better for me in an area that .net is lacking and I'll learn it. Show me that it's better in all of the areas I care about and I'll master it. That will be a hard sell. .Net is a fantastic platform; well organized, discoverable, consistent in quality, exhaustive in scale, performant, and still growing.
Will .net last forever? I don't know. Older Microsoft technologies are still around and .net is still in active development. I don't see Microsoft giving up the massive investment they have made in .net any time soon. Whether or not Microsoft stays relevant in web programming is up to them, and up to those of us using it.
As to startups using .net, I've worked for two. One provided a product that allowed you to order print ads made for your franchise. It accepted photoshop PSD files from designers, took them apart and allowed the ordering party to swap layers and text to fit their business, all with a WYSIWYG live-previewing interface. I wrote the original system myself in 2004 and the company is still operating today. The second was an online ad distribution company. The system I helped build was capable of targeting, serving, and tracking over 500 million ads per second at peak (a major accomplishment in 2008). The company is still operating today. I was not told what the companies did until after the interviews and pay negotiation. There was no way to know before I accepted the jobs whether or not they were "cool" jobs. Before starting my own business I was invited to work on a high frequency trading system built in .net. I turned it down, but they are doing very well. I've worked more for large businesses and mega-corps who also wouldn't tell me the nature of the job they were hiring me for until I was hired. How do you know ahead of time if a .net job is cool when all you hear about it is “Senior .net developer needed”?
Is there a lack of marketing from MS extolling the values and virtues of .net on the server (especially for start-ups)? Yes. But that's always the case with Microsoft. They are terrible at marketing. They will always be terrible at marketing. Should you be using .net at your start-up? That’s up to you. I’m using it for mine.