OP checking back in. Thanks for everyone's replies. Sorry that mine is so long.
I lament that I brought up the comparisons to the capabilities MiG radar, because my argument was never about realism. It was an argument for better, or perhaps... at least more diverse... gameplay mechanics for combat. I will try to explain, with examples.
Suppose with great sensors you could detect larger ships at 100km, but smaller ships were only detectable at 20km. Suppose again that all of those distances are inconsequential in asteroid fields, because you could avoid detection by hiding behind the asteroids. Now you have the scenario that a smaller fighter could, by hiding in the asteroid belt, sneak up on a larger ship with long range missiles. The fighter, using cover, could approach to within 2-3 km before it was detected. So if the fighter wished to engage the larger, better-armed ship with long range missiles, it could close the distance before being detected to completely negate that missile advantage, and then engage it's prey where it has the advantage in close range maneuverability. In this case, despite the long range weaponry, you still end up with your classic close-quarters ED dogfight.
People above keep calling missiles "fire-and-forget". Hardly so! Modern missiles are very defeatable with chaff / flare / ECM / evasive maneuvering. If long-range missile are still too effective, then make them semi-active (meaning the target lock must be maintained until they hit the target). Now the target aircraft can break missile lock using ECM, or again asteroids, or possibly other gameplay mechanics. In jet aircraft combat, it appears that many of you guys don't realize the missiles can start flying at 20nm, but they are definitely not guaranteed to hit their targets. Often, the first volley of missiles is defeated while the fighters continue to close range and they still end up in a circling dogfight. If this wasn't true, they wouldn't be building thrust-vectoring jets or putting guns on jet fighters anymore. So again, even in open space with no cover at all, you have the potential for the engagement to end in a classic ED dogfight.
How about getting interdicted? Immediately a close range encounter. ED close-range dogfight ensues. Unless one of the ships manages to pull away. Then they could employ long-range tactics.
Somebody said it would be terrible to have missiles reach 20km, because people could detect the missiles and just initiate a jump to supercruise to avoid getting hit. That's exactly what I think would be awesome! If you were the aggressor in that situation, you would have to consider that possibility and close range before engaging.
How about closing on a pirate and getting a warrant scan done at ~5km? ED close-range dogfight ensues.
What about technology's role in a larger combat envelope? Give players stealth hull coatings at the expense of durability, so they don't get detected at those larger ranges. Or different sensor options with advantages and disadvantages at various ranges. Even if weapons were effective to 20km, there are ways to keep the battle from always being a one-shot kill at 20km.
Can you guys see how much more diverse the combat landscape could be if there was more range involved? More range = more time to react = more options = more diversity in combat. Again, my point was never to argue realism. Of course if we want to talk about what's scientifically plausible, we are talking about combat distances of 1000's of miles. I just think that combat could have been much more diverse if different weapons had different ranges and/or capabilities. As it is now, they are practically all the same in range capability and so ALL fights are close-range dogfights... thus all feeling the same, and thus combat gameplay is repetitive. At least that has been my experience.
I truly think ED has the best space sim experience, with the best environments, but I think there is a huge missed opportunity for more interesting combat in those environments. Going back to my initial post... imagine some of the scenarios I put forward above being employed in this environment. Imagine trying to stealthily sneak up on your larger prey, knowing that you could spoil the attack if you were detected, especially if they have longer-ranged weapons than you. I just think that would be a lot of fun. And, most importantly.. different!