I can't quite see the difference in 1000 feet between (say) 25,000 and 26,000 (which has been around for decades) and 32,000 and 33,000 (which has been around already for a few years now). Altimeter technology has come on apace, so that's not an issue. If wake turbulence is the issue, then why wasn't it an issue in the umpteen decades of aviation up to now?