There's probably no wrong answer with this topic and you're both right. In some respects it's more of a challenge to stick with the same instruments, but perhaps it's also an equal challenge to include and integrate new sounds and actually make it sound like you know what you're doing?
Even though technology evolves fast, there's plenty of ambient-type music that doesn't sound old to these ears, or at least, still is listenable and enjoyable. My point is that though some artists may feel the demand for their sound to be fresh, it doesn't really mean that the older stuff sounds that dated. Vangelis' Blade Runner comes to mind, along with many of the early Hearts of Space and Hypnos releases, which the latter runs nearly 15 years and the former even older.
In defense of ambient/space music not necessitating the rapid evolution, you can point to the artists that sort of keep a signature sound that hardly varies- Drone On mentioned Robin Guthrie, and there are the "retro" artists whose intention is to harken back to old days using dated technology, maybe mixing it in with newer instruments. We'll probably hear more of that as ambient gets older, just as rock borrows from blues, reggae, techno, and whatever else springs up along the way.
Also in terms of creativity I'd say though it's true some artists definitely have what most would consider their peak years, they also tend to have certain waves or cycles. In those times the artist such as Steve Roach gets on a certain "kick" where a few albums have didgeridoo or electric guitar, explores that sound for a while, then gets on another kick, sometimes with newer technology. From being on this forum for awhile, I've noticed one wave with Roach is just as you think he's done and the creative well has dried up, he starts a new direction and renewed interest...