I believe that the reason the chose to close the source is just security theater to demonstrate to investors and clients. "Look at all these FOSS projects getting pwned, that's why you can trust us, because we're not FOSS". There is, of course, probably a negative correlation between closing source and security. I'd argue that the most secure operating systems, used in fintech, health, government, etc, got to be so secure specifically by allowing tens or hundreds of thousands of people to poke at their code and then allowing thousands or tens of thousands of people to fix said vulns pro bono.

I'd be interested to see an estimation of the financial value of the volunteer work on say the linux or various bsd kernels. Imagine the cost of PAYING to produce the modern linux kernel. Millions and possibly billions of dollars just assuming average SWE compensation rates, I'd wager.

Too bad cal.com is too short sighted to appreciate volunteers.

Is there such a thing as a closed source program anymore?

Look, tech companies lie all the time to make their bad decisions sound less bad. Simple example: almost every "AI made us more efficient" announcement is really just a company making (unpopular) layoffs, but trying to brand them as being part of an "efficiency effort".

I'd bet $100 this company just wants to go closed source for business reasons, and (just like with the layoffs masquerading as "AI efficiency") AI is being used as the scapegoat.