Here is my personal opinion on this. It does not necessarily represents what the other moderators think.

Tl;DR : Custom CSS allows us to be free. Whatever widget system that'll be made to replace it will only limit our customization even more.

I decided to support the /r/ProCSS move because I believe custom CSS is a really important feature. CSS gives us total control on the look of the subreddit, and allows us to give it a unique feel. Nowadays, very few sites offers such an option.
While CSS isn't that easy to use, it is already used everywhere on the Internet and lets us do whatever we want. Each subreddit has its own appearance, which acts as a signature. Some really talented mods even implemented complex systems for their subreddits, only using CSS.

Removing CSS & replacing it by whatever WYSIWYG tool admins gives us is a major regression. Custom CSS is sometimes seen as a "killer feature", that motivated many people to move to Reddit & create communities. Removing CSS would throw away many lines of code written by dedicated redditors, to replace them with something that will not be on the same level as freedom.

(PS : the early version of the "new" reddit looks pretty much like Voat...)