Can RVSM safely be further reduced?
(ICAO)

In cruise the wake turbulence levels off about 900 feet below the cruise level, which is just enough for another plane flying head-on 1,000 feet below to miss it or encounter the mild part of it. Encountered wake turbulence in RVSM should be reported to ATC, because the wind also affects it, and then the ATC will advise on a deviation for the remainder of your route and for the other aircraft behind you, for example.

Regarding the projected increase in travelers, new airways can be utilized, more use of free route airspace, using bigger (but not too big) planes, as well as improving the throughput of the runways using various technologies (NextGen and SESAR for example).