Have speakers really improved within the last 20 years??

Use your ears. If you don't hear a difference between speakers manufactured decades ago and some of what's available now, count your blessings. You'll save money.

The answer unfortunately is no. 

By the late1950s the main loudspeaker technologies had all been invented.

Sealed box, open baffles, horns and even transmission lines were all in existence by then.

Dynamic drivers, dual concentrics, paper/ polypropylene/metal cones have long been in use. Electrostatics, ribbon tweeters etc have also been in use for decades.

So what about the improvements? 

I think, if there are any, they've mainly been centred around tiny incremental engineering improvements and the use of software to juggle and hopefully hide the various performance compromises that all loudspeakers by necessity are subjected to.

You'd like to think all modern speakers must measure better but there's little evidence to support that.

The new KEF LS50 Metas are still a dynamic driver in a box, but a highly refined one that offers a performance never previously available at that price point.

Are they a huge step forward or a small incremental one? I don't know, but the speaker I'd want to compare them to would be the LS 3/5a, a renowned long time standard.

However I am willing to bet that the KEF is the far cheaper to buy if you were to adjust the BBC design for inflation.

Twenty years is too short time period. Question should be thirty or forty years. My most memorable sound experience comes from stacked Quads 63’s with Kef black boxes passive subs in the late 70’s. Playing Little Feat Columbus live. And miss my Beveridge towers...

They don't make them like they used to.

“This seems more like someone who dislikes the audiophile hobby in general”... precisely.  
His hobby is trolling, not audio.  He’s quite good at his hobby