In a fantasy version of my neighborhood, I'd get from my house to the bus stop by setting the sniper up on overwatch in a tree (with climbing spurs maybe), while the monk (in company with the Lore Bard) sweeps the sides of the road and around corners looking for hostiles, and flushes them out with her own missile fire.
Even in such a contrived situation, you haven't solved the basic problem of identifying the enemy as something you want to kill while you remain at a distance. You can see hundreds of feet ahead of you, sure, but how close does someone need to be before you recognize that they want to attack you? And if it's a
person, then there's a non-negligible chance that they'll want to talk to you first, because they haven't decided whether or not you're an enemy yet.
And even if you
could magically identify hostiles, you can't move on while your sniper is still in the tree, unless you're planning to hire a new sniper ever 600 feet. This ambush scenario you've set up has zero applicability to anything, because you don't know where or when an enemy might appear. You don't even know that they'll appear
ahead of you; they could just as easily appear to either side or from behind, where you probably
won't notice them 300 feet away.
Secondarily, consider that you are nobody special in the game world. If it was possible to figure out a simple procedure and eliminate most enemies from a distance, with minimal danger, then you would have fallen victim to such by now. If you want free victories without bothering to play through them, then it's equally fair for your PCs to be dispatched off-screen.