Genuinely curious why not pay for fantastical? It has basically the same widget, plus an iPhone and Mac app for a 1/3 of the price?
Not to be mean, but this seems like it only got traction because they got accepted to YC on another idea, otherwise would have been lost in the sea of dozens of other much more full featured or free calendar solutions?
For context, Austen Allred is the co-founder and CEO of Lambda School, so efficiency wins are going to be pretty valuable for him.
Also, as a CEO you are constantly thinking in terms of ROI. It doesn't matter how much software costs if it pays for itself several times over. This spills over into personal purchases too.
(BTW, big fan of what Lambda School is doing Austen.)
It's not really hard to make my life $10/month better, and who knows how many people that is true for? I wouldn't overthink it.