From my slumming, used gear seems to be wildly overpriced, but what people are asking and what they are actually getting might be two different things.
Generally, tube gear from the '50s through '60s is easier for most people to update, because much was hand wired to begin with, and consequently it is easier to get to whatever might need replacing. With some tube gear, especially Dyna, active third parties continue offering complete updates for whatever you might need, from electrical parts, to new chassis, to revised DIY circuit board designs. This makes those ancient devices practical for current use. A thoroughly updated Dyna amp or preamp could likely measure better than a NOS period piece.
Solid state is a mixed bag. Some companies used proprietary parts ( for instance, ARC 'analog modules', various unobtanium FETs and such); parts that are no longer in the supply channel. If you can't get the part, the only option is to scavenge another period piece and then try a swap. But in that case you remain stuck with something old and questionable, from a reliability standpoint.
SS mass produced circuit boards tend to be difficult to get to, they may in fact be corroded or brittle, and individual parts might be difficult to replace because of their location and density.
As far as 'other' reviewers and what they review? The reason they might not talk about the old stuff is that many reviewers are essentially a branch of the marketing arm of the hi-fi industry. It's not their job to keep the old stuff in circulation. That, plus they know which side of the bread is buttered, and act accordingly. It makes them almost worthless as a source for consumer education, but that's that.
One point:
Hi Fi News occasionally reports on vintage gear, providing new (generally cursory) measurements. It is not clear to me whether their newly provided measurements are done on 'old' gear that has been 'updated', or gear that has remained stock over the course of its history.
www.hifinews.com