YC has 100+ companies in a batch and funds several hundred new ones every year. It should be obvious that you can't draw broad conclusions about work culture or anything else after working with exactly 2 of them. In fact you can't draw any kind of conclusions at all because they are all going to operate in their own way.

Yes the grind and hustle mentality is alive and well (and is hardly a new thing), but I have also personally worked with a "hot" silicon valley tech startup where all founders were 40+ and everyone went home to their families at 5pm, and they had a multi-billion dollar exit.

On your side the best you can do is find an environment that fits you, have the company make the expectations clear up front, and negotiate accordingly.