This is denial. There are absolutely problematic side-effects with the existing calendar system, and the obvious use-cases that jumped out to me are on the business/financial side. Yes, the changing length of months makes financial planning slightly more difficult. The arbitrary nature of how many non-business days occurs in a month cause issues (some months can have many more 'business' days than other months, skewing financial data for those months and the months surrounding them). Don't get me started on the fact that different years have, in the US, different lengths of time between Thanksgiving and Christmas, skewing consumer behavior.

I'm not saying this is a good enough reason to even begin trying to upend the existing calendar system and replace it. But the somewhat random nature of how weekends and certain holidays are distributed throughout the calendar can have effects on business, and presumably other areas. One might suggest it is all awash because the bad effects of one year/week/month will erase the good effects of a future one, but I don't find that comforting if it contributes to, say, the company I work for having an under-performing year, resulting in layoffs (granted, there would be other factors here as well). Regardless, the inconsistency is a problem.

I recall working at a FinTech company and hearing some team gloat about something like an 8% increase in MoM revenue, only to be shut down by someone in finance pointing out that there were 8% more bank days that month. Imagine someone wasn't thinking of this and people got promoted (or fired) because of a total nothing-signal being misinterpreted as a hugely positive (or negative) performance. The existing calendar allows this.