Great point, Onhwy61, and yes, it certainly is ironic.
There are certainly a multitude of factors that are involved. When it comes to classical music, at least, I suppose the most significant factor is not related to equipment quality per se, but to how it is used. When the classic RCAs, Mercurys, Deccas, etc. were recorded, a minimal number of well placed microphones were used, and the signals they provided were minimally and simply processed. Today the norm, especially for symphonic recordings, seems to be the use of a forest of microphones, placed too close to the performers, and for their outputs to be heavily processed with elaborate consoles that are adjusted by misguided, artistically insensitive, and/or incompetent engineers.
That said, I'll also particularly second the response by John (Jmcgrogan2). Having fairly extensive experience with vintage tube gear of the 1950s and 60s I can say that many of those pieces from the better manufacturers of the day, if well restored and/or in top condition, and if suitably matched to the associated components in the system, as a general rule of thumb will easily be competitive with modern gear selling at comparable price points. (I'm referring to the present day selling prices of both the vintage and modern equipment).
I'll say also that it is not just collectibility and rarity that drives the present day value of vintage Marantz tube gear, Tannoy speaker drivers, Brook amplifiers, REL tuners (no relation to the British subwoofer manufacturer), etc. Nor are those the only reasons for the values of 1930s(!) Western Electric speaker drivers, amplifiers, and tubes, which in some cases approach the astronomical. (And one of our most highly experienced and knowledgeable contributors here, btw, whose system is definitely in the upper echelons price-wise, makes very extensive use in his system of some of that 1930s Western Electric equipment).
One final point, which Ralph (Atmasphere) has made in the past, and which I think is very valid. Those classic recordings were made many years before expensive audiophile-oriented cables came into being. And the cabling used in the halls in which many of those recordings were made approached perhaps as much as 200 feet in length. And most cable effects are proportional to length. So how could the recordings be so good? Because those cables were balanced AND were driven from low impedance outputs having very hefty drive capability. Which Ralph has asserted (and it makes sense to me) results in cable effects and cable differences being essentially (and perhaps entirely) negated.
Best regards,
-- Al
