Centuries ago, a disastrous first contact with the Klingon Empire led to decades of war. It was decided then that we must do surveillance before making contact. It was a controversial decision. But I believe it prevents more problems than it creates.
Yeah, I don't really have too much of a problem seeing Klingons in an early time frame, but it was still a mistake to include them in the pilot and keep them so close to home. Centuries certainly implies more than one century, even if exaggeration. But the implication by Picard is that the option for surveillance was there. This can't really happen in the situation of a Klingon landing on Earth and Vulcans holding our hands. Otherwise the Klingons were a bad example for the point Picard was trying to make.
Also, does decades of war include skirmishes and cold war? It seems as though there was never quite a full scale war with the Klingons. If so, and if centuries implies at least two centuries had passed, there was probably more than simply decades of war.
It's best to think of Star Trek as fantasy, not sci-fi. By the end there it was just as far-fetched as Star Wars, only with an exponential increase in babble.
It may not be hard sci-fi, but it's a
lot harder than Star Wars. Star Wars gave no thought at all to the workings of its technology. It was essentially magic, though based on real sci-fi. Plus, there's the force. There isn't really anything that fantastic in Star Trek that would pull it more towards a fantasy label.
Star Trek definitely tried to at least give some credibility to its technology. While there was a fair amount of babble, there was a lot that was based on real science. Yeah, the Dyson Sphere was pretty disappointing, but that doesn't negate everything else, and certainly isn't equal to Star Wars' complete ignorance of science.
Given the ideas of inclusion of real stars and real scientific theories, I don't think it was too hard to expect them to stick within some of that instead of crazy warp 10 lizards or other far-fetched stuff. I think some writers just didn't care as much as others.