Mockup Quality to Justify Real Orchestra
The quality and also availability (more sample content creators, smaller price tag) of orchestral samples has increased dramatically over the last 10-15 years and while back then they were easily to distinguish from the real thing, by now a skilled sample programmer can fool 99% of regular audience and even a big share of professionals to believe to hear a real orchestra if the music is limited to things that samples can pull off well.
Still, an AB comparison will reveal a subconscious difference for most people. A real orchestra performance creates more emotional impact, feel more homogenous and more organic than the best mock-up of the same piece and particularly in a multichannel environment will sound bigger and broader.
Another part of the truth however is that we can observe a decline in the value that filmmakers see in a real orchestra. Most of them being part of the group that can easily be fooled by a good sample mockup, discussions about whether it needs the budget to record a real orchestra if that demo sounds great already have increased as well.
One strategy with demos is trying to find the thin line between “selling the cue” to the filmmaker while still leaving something to be desired to be filled by the orchestra session. This could either be done by consciously picking sample libraries that have weaknesses or by doing sloppy programming or by writing music that brings samples to their current limits (frequent articulation switches in cohesive lines, playing techniques barely covered by libraries, lots of runs etc.)
While the latter one is probably the strategy that justifies a real orchestra most strongly, it certainly is not useable all the time. If your director demands long sustaining string chords, this strategy is useless. The bottom line here is to be strategic about how well you produce a demo mockup if you want to have an argument for a real orchestra.