Wow! That's quite an over-generalisation, or so it seems to me. I will grant you that by the 20th century, no professional composer had an excuse for not knowing the techniques of instrumentation and orchestration - the books by Berlioz, Rimsky, Strauss, and the scores illustrating their techniques were all readily accessible to anyone with the inclination to study them. Yet you still find examples of composers writing unsympathetically for certain instruments; I'm reminded of statements one occasionally reads, such as, "Did X loathe having violin lessons in his youth? He seems to have a hatred for the instrument, judging by the unsatisfying and difficult lines they are routinely asked to play" (my paraphrase of the general intent).Yagan Kiely wrote:Mozart, Berlioz (not Beethoven - arrange him! make it better!), Strauss, Wagner, Mahler, almost all 20th century composers did know a considerable amount on instruments.
In which I think you are making the rather large assumptions that the composer has written a piece that is idiomatic for the instrument(s) in the first place, and likewise that the arranger has not attempted to adapt the work to exploit the qualities of the different instrument(s) used in the transcription!However, no matter how much they try, unless they re-compose the piece, if the composer did specifically write it for that instrument they aren't going to effectively arrange it, regardless of how knowledgeable.
I gather you're against the more-or-less "straight-down-the-line", uncreative transcription from one instrumental designation to another, as opposed to the more creative re-composition of a work to suit the differing instrumentation: wouldn't it have been fairer of you to make that distinction, rather than dismiss all arrangements out of hand?
Admittedly making arrangements is often treated as a routine task undertaken when a given piece of music for a specific type of ensemble doesn't match the instrumental line-up that a certain performing group has available, but even in that case a skilled arranger will be able to add something in the process, rather than drudgingly executing a mechanical copy.
Regards, Philip