Some people see a mixed neighbourhood and like it, see the mix as a single style...
That's a really good point. They want to preserve the current mix, which they regard as a unified and complete whole. It's understandable to want to preserve something that you like. But it's also absurd because the mixing is still going on. It's not as if the mixing paused at this perfect moment, and now it's threatening to start up again. The mixing is always still going on. How can you preserve a moment in time? You can't.
The same thing happens downtown. Every new building somehow represents a threat to the current mix, even though the current mix was developed over ~150 years and contains an enormous amount of variation. Big bites were even taken out of it here and there. In some places the bites were filled back in with other stuff, and in some places the bites were left open. The evolutionary process continued year over year and decade over decade. And yet somehow a single new building proposed in one corner of downtown represents a threat to the mix as it currently stands. As if the mix as it currently stands was the end goal of all of that endless reconfiguring. In fact, there never was an end goal.