Maybe it is a change of times, but when he got into the detail of why he was quitting, it was about the hard work ahead, which is rarely fun for most people.
I don't feel sorry for him at all. He found something that was of interest to him and he decided to head down a path towards that interest. Good on him. Kudos to him for giving it a go. However, the problem he has made for himself is that he went public far too soon and is now publicly saying he is quitting because there is too much hard work ahead for him.
Startups have a low rate of success and is about testing an idea that has a good likelihood of failure. Different proposition to start with. The more appropriate comparison is making some item that can be useful to you but has a lot of tedious and fiddly work to be done. In his case it was a computer language which always comes with implementation problems to do deal with. One knows this up front, or at least, should know this.
The problem is, he hasn't pulled it off - there is still a long way to go. One of my areas of interest is language design, compiler construction of virtual machine implementation. This subject matter is highly complex, tedious in many ways and requires a great deal of patience and perseverance. For each problem you solve, more arise to be solved.
For an individual, this can be daunting when you finally realise that you have undertaken a massive project. It is especially so, if the language that you have designed doesn't strike a cord in other people. But if you have a goal in mind, then continue on even through the tedium that will come.
Things will always get in the way of you completing side projects, that's life, but you can still keep chipping away as the years go on.