I don't think the guys are saying you have to replace everything, but its always worth securing your investment by pulling the bottom apart & checking things. If the crank is straight, bearings & inner oil seals are good, & everything else falls into specs, by all means, clean it up & put it back in. If they are not in spec, or you find a problem, your engine might not last long. You would just be miserable if you stuck $400 in the top end & two months later your engine scatters from bad crank end seals, rod bearing, etc.
New cylinders are expensive. Usually a machine shop can bore them for under $75 each. If your crank is good, you might be able to go through the engine & spend around $500 or so. But the reality is, it needs to be pulled apart to see what you need first. who knows, you might get lucky & not need that much.