To come back to my snippet: it's an excerpt from Tchaikovsky's 6. symphony (first movement, just before the development section starts). You were right, the first snippet is my mock-up, the second one the recording. If you listen closely, you can hear that there's a big difference between the two when the bassoon takes over the line from the clarinet. In the mock-up, the transition is smooth and the bassoon plays it even a little more quiet than the clarinet (as it should be according to the score). In the recording however, the bassoon sticks out like a sore thumb. It's much louder than the clarinet instead of even quieter.
Now, there's the problem. You could actually say that the mockup sounds better in that aspect. But it's not 'natural'. A bassoon can not play as quietly in it's low register, whereas the clarinet can play very quietly over it's whole range. That's why this particular motiv is almost always played on a bass clarinet instead of a bassoon (eventhough Tchaikovsky asked for a bassoon).
Anyway, here comes a problem for the 'natural volume' feature. If you wanted to restrict your sample instruments to only what they can play in reality, VSL would have to delete the pp layer for the bassoon's lowest register. So for example, when playing in the bassoon's low register with a volume of, say, 22, there should either be now sound at all or one that is much louder than the requested. I can only imagine the complaints VSL would get if that was the case. And that's but one example. If VSL applied this logic rigorously through all instruments, their libraries would be very inconsistent and tedious to learn. You'd always have to ask yourself: 'oh, why can't I do that particular thing. Ah, yes, it is because this instrument can't play a trill on these two notes'.
And another thing why 'natural volume' can't account for each and every situation. In a real orchestra, the players listen to each other and balance their volume and pitch accordingly. When a horn section of 4 players has a ff marking, in one case they will play with all their force at 120 db (for instance when there are a lot of other instruments around, all playing very loudly too). In another case however, despite having the same marking of ff, they may only play at 90 db (when they are playing in a smaller setting, for example, or when another instrument has the main line and should be heard clearly over them). That's because dynamic markings in a score are no scientific instruction, but always depend on the context. And the 'natural volume' VSL provides can never respond to context. That's the users job, really. Despite your claims that 'natural volume' is a measurable, invariable quantity, it isn't. The balance between the instruments depends on the context. Like it or not, but you yourself will have to take it into account and adjust some things accordingly.
Software can only get you so far. There's no way around learning about the individual instruments you are using if you want a 'natural' sounding orchestration, and balance. So I'd advise to pick up a book about orchestration and learn the basics. That way VSL's 'natural volume' will still be a timesaver (because you don't have to set up everything from ground, but only tweak a thing or two depending on the context of your music), but you won't depend totally upon it. And you could use whatever sample library you'd want, not just the ones with 'natural volume'.