The Vienna Symphony Library is available today and can essentially replace an orchestra to all but the most discerning of ears.
Here is an example of the E.T. theme. There are a couple of parts where I can tell it's a bit artificial sounding if I really listen, but it's approaching the flawless threshold.
That said, there is a particular order of ease of simulation: percussion (including piano), strings, brass and woodwinds. The latter two are notoriously difficult to emulate because they are so closely tied to non-discrete complex forms of movement of the mouth (articulation). For example, see
this demo of one of the betters saxophone emulators - still something missing even to uneducated ears, but not too bad in a mix. Strings can also be difficult to emulate, but if apps from companies like Prominy are coming out,
guitars and
violins, this is getting scary.
There are a couple of serious implications of this. First and foremost is what the value of a live performance is with and without musicians, which the linked article addresses. The second is decreasing numbers of people willing to learn these instruments. For a lot of folks who compose for small-budget TV and movies and can't afford musicians, it's a great way to go. Nevertheless, it's the same cautionary tale as the decline in handwriting that coincided with the rise of computers with keyboards. You can't replace handwriting in a lot of circumstances.