I've never heard of YC pushing founders to do something with their startup that the founders didn't want to do. Perhaps there have been corner cases but it's definitely not in YC's culture.
Id assume it's more of an unspoken agreement and perhaps more in the heads' of the founders than YC. The degree to which both parties are conscious about it likely doesn't matter--the point is it's likely exactly what happens. Young founders come and get addicted to a train of mountain climbing in the name of the company, while potentially sacrificing happiness and control of the company. It's all very psychological and ego based--where young founders are driven to be famous and successful overnight--when a more patient route, eventually landing on doing something they are truly emotionally invested in, would be a better more psychologically balanced option.
I imagine more mature founders are confident enough to realize they can get past the "hardship" of starting without sacrificing equity until deeper in the process. If ur building apps, and ur a developer which u should be, and u have been doing it for 10 years--and ur worth ur salt--u don't need YC to get up and running. Younger coders living in their parents house who have it even easier often fail to recognize that for the prospect of Silicon Valley big wigs giving them pats on the back, and that's the problem here. It's basically akin to recording labels signing young dumb talent who don't recognize their capabilities and options. I know this first hand--as am I one such person having gone through these progressions. I'm 31 and used to dream of YC and all these people funding me. Now I don't want a single soul funding me beside myself and I've aligned my interests and my development skills with a concept I will be doing for the rest of my life. And I certainly don't want anyone in control of my own destiny but me.
There are other formulas for success with software, especially if you are deeply merged with what it is that u r doing, rather than just glomming on to the first idea for its potential to make money or create impact, then convincing yourself ur super passionate about home meal delivery. There are people who have become billionaires 30 years down the line for sticking to their guns and doing what they loved. In short I find the YC model extremely shallow and results in lots of young people ever eager to feed their ego with whatever idea will get them financial success, customers and "impact" as soon as possible. Because of how hard it is most get punished for their greedy ego-oriented thinking, ie through the pain of pushing such a thing through to success. Those that manage to win have no guarantees they are happier for it. It's not the success that makes one happy, but how u go about it. Too often than not it requires negative choices--just like Kalanick; I can't help but imagine his internal world being anything less than a greedy STRESSFUL never ending paper chase. Indicators of one's own internal world is what's going on around them, and just look how dark and ugly it's been getting for Uber lately. None of that sorta stuff is love and happiness. Paul Graham is a nice guy, but id urge him to question the concepts he's putting out into the universe. YC perpetuates ego ambition, not necessarily healthy growth. Good for all u guys, ur all rockstars from the narrow perspective of startups. Now achieve the same in ur personal life and in consciousness of ur own psychology and intents. That's far more important and our true tests in this life. That's not to say material success isnt important, but only up until it becomes a distraction from mastering your own true intent.