How do I objectively know if I should commit to it?
- could you work for them and make them better with a better payoff? (I see they only officially have a Customer Success Manager opening in Hungary right now - but most companies will hire someone who'll make them better even if the opening is not "officially" open, presuming the expected payoff is high enough)
- how long will it take for you to build your "single feature" competitor?
- do you have any [potential] customers lined-up to try your new service?
- can you (as a single developer) handle any/all support, sales, etc issues that will arise if your service gets used (before you get enough money to hire more staff)?
- do you have any other differentiating factors in what you want to build vs what is available - locale, industry segment, etc?
A business requires both.
The fact there's a competitor actually reduces your risk because it shows the market does exist.
This is all you need. Continue to iterate on the idea. You haven't even made it yet; maybe you'll never finish it, who knows. You are way too early to be thinking about competitors. Just have fun working on something you like.
I was chatting with the COO a few months later and he said that the key is that you just have to be "1000x better" (exaggeration for emphasis) than the competitor in some key way. In the case of Calendly, the problem is how clunky it is.
The gist of it is that yeah, you can't compete against the multi-million dollar incumbent directly, but you can do one thing way better than they do and that might be enough to pull away some customers.