There are several things to consider here:
As siwi said, it is wrong to assume valve horns have made horn playing easier. They have made certain -specific- things easier, such as chromatic play, producing equal timbres for all notes, and greater flexibility in the lower range.
But you have to consider that a modern hornist on a double horn (while having a lot of tunings available via valves) is still mostly forced towards the lower tunings, as the vales only ever lower the base tuning.
This means that the high range on an ordinary double horn is always more problematic than on natural horns, which why most professional horn players, when playing baroque/classical music in high registers, will either:
1. be really good at the highest range.
2. Play it on a descant horn (high F) or a triple horn.
3. Play it on a natural horn. (I'm not sure how it is in the U.S. but over here most period music is played on natural horns anyways. Most hornists in the greater orchestras own a natural horn and can play it.)
4. Or even: leave it to people who are specialized in music of that time, and happily continue playing their Strauss :P
Playing baroque and some classical music on an ordinary double horn can be tough because of the common high range, and yes, it's often going to cause problems. (Both because modern horns are built wider, and because the natural tones in a high register on lower base tuning are so close together that you'll miss notes much more easily. And you often -have- to use such lower base tunings, because modern horns aren't really built for too elaborate stopping techniques, so you'll -have- to use your valves.)
While I don't own a natural horn (they're so expensive :() I've played them plenty of times and it's simply awesome how easy certain stuff is on them. (Plus, I absolutely love their sound.)