To me it is utterly silly to think all the great conductors lived inconveniently just before recording technology had developed to a high level.
I'm going to shoot past the absolute of "ALL the great conductors' and address the general point. In the early 20th century, certain types of music and certain performance styles existed that just don't exist today. For instance, try to find a current conductor who works like Stokowski. Also, certain types of music were more prevalent and popular in the early 20th century. The most obvious example of that is opera. Opera was everywhere and ordinary people played it on the piano in their home, sung arias themselves, and had phonograph records of Caruso, Galli-Curci and Melchior. These singers were of the highest calibre because opera was a big deal all over the world. Opera performers travelled and performed for huge audiences. There was competition between performers to excel. The standard was clearly higher than today when opera companies are going bankrupt and great singers are going into other fields of singing. Another factor is orchestras. Every town had a a live orchestra. Many had dozens of orchestras playing all kinds of music in hotels, nightclubs and theaters. Think of the number of great orchestras you see mentioned on early recordings. Only a fraction of them still exist, and several of the ones that do are a shadow of their former selves. Again, symphonies today are going bankrupt. There are great musicians, but it is a lot harder to make a living as a professional musician today.
Now, if you want to qualify your statement to say that it's utterly silly to say that period performance baroque music was better in the past than today, I would totally agree with you. There wasn't a lot of that back in the early 20th century. There is more deep catalog repertoire and avant garde music being played now too. But in core repetoire it's hard to argue that any conductors today are the equals of the big three... Stokowski, Toscanini and Furtwangler, not to mention Walter or Monteaux. The amazing thing is that all of these conductors were working at about the same time. Today we get recordings of Beethoven symphonies because they always sell. But there isn't a lot of difference between the interpretations. But Stoki's Beethoven was nothing like Toscanini's and Furtwangler's performances varied wildly from one to another.
Recording had a profound impact on the performance practices of classical music. It encouraged researched perfectionism over bold experimentation. It brought a small number of orchestras into everyone's homes, rather than every town having their own orchestra. It began the decline of the superstar conductor. And since a single performance could be played any time any place, it reduced the amount of live performances, thereby narrowing the field.
There are historical recordings for which there is no match today. And there are great modern recordings. Generally, they are different types of music so you can't compare them. In my collection, I have great recordings from 1900 to the present day. When I choose a CD to buy, it isn't because it's digitally recorded or an SACD or stereo, it's because of the performers and performances. I choose the best from all eras. And I have an understanding of different interpretations. I don't have a rigid idea of what a "proper" performance of Beethoven's 9th is. When there is a proper way to make music, I think you can safely say music is dead.
Classical music listeners keep talking about Szell like he was a semi-God. I don't think I have ever even heard his conducting. Someday I might listen just to know what the fuzz is about.
Szell was very similar to current conductors in that he stuck closely to the instructions in the score and didn't allow himself to elaborate on it. Toscanini was the first to do that, but he had an energy that was very unique. The opposite of that approach is Stokowski who was called a magician because he would sculpt the performance with his hands as he performed. He wasn't afraid to go off the score and interpret for himself. That is what is missing today. In the past, the composer and performer were equals. Both brought something to the music and it wasn't complete if they didn't add something of themselves. Today that is allowed for instrumentalists (following the lead of Glenn Gould) but that isn't allowed for conductors any more. They have to stick to the script... the score: nothing more, nothing less. That change was the result of the impact of recording. Everyone now looks for a "definitive" performance they can capture on disk and play over and over. In the past it was about an etherial ephemeral performance that captured some special aspect or mood in the music that had never been expressed before. I miss that.
Historical recordings form an ocean of great music that doesn't exist in any other form. If you limit yourself to just modern recordings, you are doing yourself a disservice, just as if you only watched movies since Star Wars or read books since the 1970s. Generally, the people who say, "Who needs a historical recording of a symphony you can get on CD?" don't know much about historical recordings.
