So if we reach 90% of segwit activation, we could vote that it's not 95% anymore but 90%? wouldn't that piss some people off? I don't get it so I would like to know.
If 90% of the miners decide to orphan the blocks of the remaining 10%, then they would have 100% consensus. It isn't that we can vote to change the threshold but rather that the threshold can effectively be lowered if a majority of miners can orphan all the blocks of the miners who are not signalling the change.
Also, has a 95% of a big group of people ever agreed on doing anything? Shouldn't this have been foreseen a long time ago? I think 95% it's too much... but at the same time, it's great to guarantee that the big majority of people want something, but that 5% could be potential trolls, so I think 90% is probably a good enough compromise.
Yes, this has been done before. All soft forks in the past have had a threshold of 95% and we have activated several soft forks already with that threshold.