I was an 8.5 holdout for a long time, all the way through the X series and the first two years of SPlat. I finally came up to rev, not because 8.5 no longer did what I wanted, but because I felt left out of the conversation.
Looking back over the 8 years since 8.5's release, I'm having a hard time naming
any critical feature that the current version offers that 8.5 didn't have. Certainly, there have been nice touches along the way. CPU load balancing, per-plugin oversampling, ARA. But none of those things actually enable me do anything today that I couldn't have easily done in 2009. Nor would any other DAW.
During those 8 years, Windows has undergone major changes. Windows 7 broke my Tranzport. Windows 8 broke my heart. Windows 10 has had its ups and downs, and I don't trust it from Tuesday to Tuesday, but SONAR 8.5 would still be fine with it. Will SPlat still be fully functional 8 years from now? Probably. In fact, probably 20 years from now. By 2027 I'll likely be on oxygen and eating through a straw, but I'll still be able to make a record with SONAR.