It's relative though.
What we saw in the OT was a particular aspect. There's nothing to say the emperor didn't know about the knights of ren then and had either largely wiped them out or had them off doing something else. The OT was Vader vs Luke for the most part. That's the aspect of it we were shown. The entire empire is colossally bigger than what was shown on screen. We weren't shown a single planet under their control really. Death star, star destroyers, a lone base on a moon - that's all we were shown of the empire. So, there's plenty of reason to not reference or show them during the OT. I mean, there was plenty Luke wasn't told about the events of the PT which we all know Obi and Yoda knew and simply withheld. Of course, we know the reality of that is the PT didn't exist and all GL had written of them was 'Obi and Anakin fought in the clone wars'. But aside from that there are plenty of reasons for the Knights of Ren to not be mentioned during the PT and OT if they existed. And again, we know that's because they simply didn't exist until TFA was written, but still. Inquisitors weren't shown or referenced in the OT but, we know they existed until a couple years before ANH and we know they existed well after as they're in Ahsoka still.
All we saw of the knights of ren on screen just made me think they were Kylo's group of yes-men or something. They stood there did nothing really. They tried to fight him a bit in ROTS and got their butts handed to them as I recall. To me, they don't really matter and maybe that's the reality. It was a group of potentially bad-@$* guys during the high republic who got obliterated and Kylo and his band of doofuses picked up the name again to try and use it for the cache'?
I'm hard pressed though to criticize the fact they may or may not be there until they've actually presented the story. I get that with the Disney stuff it's foolish to expect good writing, but it is very possible to write it in a way that it's disappearance until TFA makes sense.
I get that the Star Wars world was bigger than what we were allowed to see from the OT, which is why our imaginations ran wild with "what ifs?" for the next several years before the PT came out. That was what the magic of what Star Wars was...opened up our imaginations from a great original story.
I agree with you that they "could" write it in, and probably will but i believe it is a mistake to because , once again, it creates more problems than it solves. I believe the PT did as well, so i'm not singling out Disney in that regard but they are doubling down on their ST because to admit its failure well...cant do that...anyways, i see your point but here is something to consider. The Emperor was the top dog in the galaxy(in the OT) was he not? Based on the OT alone, did we know how long he was? We knew absolutely nothing but one fact still remained that we did know. He was the Emperor. Everyone knew of him. When he was defeated, you have such a finality to that evil, which completed a fulfilment in purpose of Vader and Luke and the rebels that, as a story, there is nothing more that needs to be done there within the confines of all who was involved. But, to your previous point about a lot of stuff happening in a 100yr period in the real world, if someone held a press conference and said, "Somehow, Hitler has returned" and the SS troops are back because they've been mysteriously hidden somehow, they'd be laughed all the way back where they came from. Since his demise, plenty of evil people have risen to power etc. but him and his troops are gone, never to return again...along with the likes of many others before them. So, if they want to go that direction yet again, then they fail once again to learn their lesson. As much as they may think i'm their enemy, if they will listen, i would be their best friend...i'm only trying to help them..lol, but you can't help someone that doesn't want it and thinks they have it all figured out.
So GL does the PT...were there problems? you bet...a lot of them...head scratchers!
Then the ST...Don't worry...I won't beat the dead Taun taun with my lightsaber
and so on...each time they hang slightly around the Skywalker era, it just causes more problems.
I go back to your point that we can both agree on. KOTOR was far enough away that it's inconsequential to the OT. I'm ok with things like that.
Disney can prove their worth by cutting the umbilical cord away from anything that disrupts the OT. They won't though...
Is the Star Wars world huge? Of course, and if i'm hearing you right, they need to go a lot further away than they are, which is why i won't sign off on really anything they'd want to do so close to the original trilogy.
Thanks for the good dialogue, btw. I like this kind.( Thanks for taking the time to have well thought out points...i appreciate that)