The desirability of older guitars really started in the late 60s/70s when it became apparent that the older guitars were better built and sounded better than the new guitars. (Or in the case of the Les Paul went out of production).
Obviously now there are collectors/ investors in the market which have pushed prices up.
Regardless, I've played ALOT of vintage and new guitars, so am able to give an opinion based on a significant sample size.
Clearly, not all vintage guitars sound/ or play great, and there are some fantastic new guitars being made. However, in my opinion, the very best vintage guitars still sound better than their modern equivalents.
Some modern Teles sound really good. However, I've yet to play a modern Strat (Custom shop, Nash, Maguire etc) that sounds give a really authentic vintage sound, and as for Gibson-style guitars there really is no contest comparing modern guitars with there vintage equivalents.
Vintage guitars don't have to expensive though. At the moment my main guitar is a 1950s ES-225T, which cost me under a grand. OK, it's a slightly quirky guitar, but the build quality is amazing, and it sounds and plays great, and is actually much cheaper than a modern Gibson semi.