"you have to acknowledge that extremists and totalitarians do exist" is a cheap rhetorical device to sneak in "both sides are the same" fallacies. Yes, of course there are crazies in any political movement, demographic, subculture, or other group. Elevating these crazies to the first thing mentioned when discussing the actual substance of the beliefs of a group is silly at best, but more frequently it is simply a dishonest distraction. This is not a slippery slope (also typically a fallacy in practice).
It's also simply not a useful type of statement even when made in good faith. It assumes all issues fit on scales. For example, there are people today who call themselves feminists, and have extremely different views about LGBTQ+ rights compared to others who call themselves feminist (notably around the BTQ+ part). I really don't know who is the more "extreme", since that implies that I am referencing an agreed upon "center" which these positions differ from, and that simply doesn't exist. There is no agreed upon "center" for anything. Both claim to be feminists, and claim the other side is wrong, and neither would consider the other either more "extreme" or more "centrist".
This is just one example, but I would argue that this holds true in nearly every case: Reducing something to a scale with "extremes" on each side mostly just shows how the speaker perceives other positions and is otherwise not very useful.
CS stuff: IMO a better mental model would be non-Cartesian. Perhaps a weighted graph, where the vertices are ideologies (or, to be more granular, people), and the edge-weights represent some fuzzy metric of overlap of beliefs.
I think that it is just as important to talk about the extremes and the bad ideas as it is to talk about the good ones. We bring up the extremes because they're there, and they can be tempting, and so we should warn each other about them. Saying what you're saying usually (but not always) is an attempt to not address the usually perfectly reasonable point one is responding to without outright defending the extremes. Extreme ideas do need to be pointed out precisely to do what you want: prevent the reasonable discussion from becoming one between two extremes.
You are completely right in the appropriate context: when talking about ideology A we should consider more than just its most standard version. But when comparing ideology A and ideology B, focusing on the extremes of ideology A while considering only the middle ground of ideology B (what happened in this thread) is more or less whataboutism.
To be clear, I'm not saying both sides are the same - that's a value judgment. But it's obviously not a one side is right and the other is wrong situation either.
