The fact you can't re-use symbols across projects. Seriously, it is a deal killer for many. You have to re-invent the wheel over and over again. Absurdly frustrating.
The problem with templates is that many times you have to combine them. And developers are used to creating their own "libraries" of objects (or "symbols") and new projects are usually the simple exercise of putting together collections of objects.
BTW, if webflow developers are limited to customizing templates they might as well use WIX or GoDaddy DIY website tools... much, much easier to use. The true innovation lies in the ability to combine and customize objects/symbols.
I can also see a whole market of objects (or "symbols" or "widgets" or however you want to call them) where instead of buying templates you could buy individual objects with specific capabilities.
There are some really bright coders out there who would happily create amazing "symbols" that anyone could use in their projects (and Webflow serve as the platform and marketplace for those -and, obviously, curate them to insure a safe source of symbols-).
Anyway... just my 2-cents (though I am sure a lot of people share in my frustration).