Sheldon Brown, for one. Even low-end Shimano components perform better than some vintage high-end components. It's simply technology trickle down. Many of the newer low end components are heavier than older high end, but that's a trade off. But I think the OP may be referring to a bike store bike, not a Wallybike.

I think it's a wise decision to make if you want reliability. Most vintage bikes on the market, even if pristine looking, usually need a complete overhaul if you want a reliable machine. And unless you have the mechanical know-how, and a stash of parts, you're going to be paying through the nose for labor at the LBS. And if you decide on some minor upgrades, like a different saddle, more comfortable bars, different stem, or different pedals, you'll be paying a good chunk of change. It's possible to find a vintage bike set up the way you want, but unlikely (unless you are very easy to please). Finally, there are hidden problem you can't see on an ebay auction. The bike looks pristine, but how are the hubs? Pitted? The headset, the BB? A new wheelset will be expensive, as will labor for headset or BB replacement.