Why is it a big mistake? Most likely they're not seeing enough revenue to keep supporting the other filesystems, otherwise why would they cancel support for them?
>Given the disaster that has been Windows over the last few years
Only if you don't come out of your bubble.
Dropbox is in a viral business, and they were the only ones who supported Linux.
Starting with Linux support, Dropbox's competition essentially completely ignores Linux. Box, Google Drive, One Drive etc all have no official Linux client. That makes it fair to say that some of Dropbox's customers are there because of the Linux support (I'm one of them). Note that a customer doesn't wholly have to be on Linux, but if for example 10% of the people at a customer could be on Linux, so using Dropbox for all of them makes sense, because of the Linux minority.
This is a viral business in that you use similar tools to collaborate with others. Dropbox did their spread not via marketing, but by word of mouth and minor storage bonuses as rewards for the spread. (Been there, done that.) But now there is going to be limiting of that spread by Linux folk affected by this.
Dropbox is now going to lose $1k+ per year because I'm dropping them as a paid personal and work customer. A tiny drop in the bucket indeed, but this will all add up. I think they are idiotic to take a unique selling point, and throw that away. I can't think of a single thing that makes them competitive with the alternatives now. The technical solution I would adopt is a more minimal sync mode when operating on non-optimal filesystems, and standard sync on supported filesystems.
This is all like compound interest. They have narrowed their customer base and made that rate a little lower. In the long term it could just turn out to be a terrible decision - my expectation.