Well, the plastic fantastics really started coming out in the late 70s/early 80s.
But I understand your question being are these new designs better than the older steel or alloy designs?
To that, I'd say "better" is really a judgment call. The newer polymer designs are lighter, less expensive, and quite reliable. However, the older metal designs are easy to shoot, generally well made, and quite reliable. Now, even as a 1911 fan, I'll admit that any Glock, M&P, XD, etc. is more likely to be more reliable out of the box than the average 1911 costing twice as much. Those same guns are also likely to be every bit as accurate and reliable as the Sig P-series alloy guns, S&W third gen autos, Beretta 92 series, and so on, often costing a fair amount more than the plastic guns.
But is that better? Well, it depends on what you shoot well. I can shoot either well. I would not consider somebody armed with either to be poorly armed. I would consider someone who rejects one or other kind out of hand or considers that other type of handgun to be "junk" to be poorly informed and small minded. A well rounded collection will likely have examples of all kinds of things, although it's up to the shooter to find and select something that he/she can shoot well.