The double spending phenomenon might be rare and depends upon the consensus algorithm being implemented but I guess it's still the most important aspect which cannot be ignored or taken for granted as it can collapse the entire system if not dealt with.

Yes, this is true. But I still think that pretty much all real blockchain developers understand this and try to make their best efforts to mitigate risks. It's not something that they might forget to include in their project, unlike all other things that I listed.

For reference, you could read this brainstorming by Dan and Jaekwon from Cosmos regarding DPoS algorithm that we already consider to be a pretty great.

That is great conversation, I've read that earlier for several times to understand DPOS better.