more than just a computer
I write orchestral music and at first I used a computer just to generate conductor's scores, but after I had templates set up I was able to hear computer realizations of the scores as well. As the technology improved I have been able to use orchestral emulations for clients (Tv and corporate mostly) who expect the same kind of orchestral score as movies, but with the budgets of commercials.
So...having worked in the trenches I know for a fact it is possible to have a computer realized score sound as real as an orchestra, but not without a lot of work! You need huge drives, and a fast throughput, cutting edge samplers and cutting edge libraries, plus you need to have both an understanding of what a real orchestra is capable of as well what your library is capable of as well. To make a mock-up truly realistic requires almost as much attention to detail as orchestrating on paper!
I guess the real answer to the question is: no computer can replace an orchestra, but a capable composer-engineer, a powerful computer, some top end libraries, lot's of time and good gear can certainly come close!
The main aspect of that use though isn't concert performances (obviously) it is movie scores and music to picture. In a lot of those cases, it is too expensive to hire an orchestra anyway, so while losing an audience or performance work seems to be a valid concern, I doubt a lot of orchestras have any real need to fear.