Ilkka Talvi gives his take on the changes in classical music and reasons for the potential decline:
One reason is the lack of composers who are able to create new music which has all the needed, critical elements: memorable melodies, engaging rhythm and pleasant harmonies and sonorities... But music has to be acceptable to our ears, not just to consist of loud meaningless sound effects.
I both agree and disagree with that last statement, especially with respect to live concerts. I don't need my music loud (any more) and I generally prefer a more intimate setting. But intimate need not mean "pleasant." What are meaningless sound effects to him could be compelling timbre to me. Having spent the last forty years stretching my ears by listening to a large and diverse body of recorded sound, for example this week, a Merzbow remix of Xenakis, the prospect of hearing a traditional orchestra playing traditional repetoire isn't enough to get me to pay for the experience (any more).
I suppose my interest in works by John Adams, Elliott Carter and Edgard Varèse doesn't qualify as traditional, either.
Finally, I have no informed opinion about performance practice per se but Mr. Talvi's oft-interesting post also includes this:
I don’t think that anyone should conduct or sing a sacred piece of music unless they thoroughly understand the meaning and symbolism of every phrase.