Serious predictions actually inferred from the progress of existing (MCTS and others before) bots, which was something like 1 stone every two years (I don't recall the details, but it's easy to find out there). Top professionals were estimated to be something like 10 stones stronger than the best bot at 2008, so 2025 wouldn't sound too conservative.
"At the US Congress 2008, he [Myungwan Kim] also played a historic demonstration game against MoGo running on an 800 processor supercomputer. With a 9 stone handicap, MoGo won by 1.5 points. At the 2009 congress, he played another demonstration game against Many Faces of Go running on 32 processors. Giving 7 stones handicap, Kim won convincingly by resignation."
(Kim Myung Wan (born 1978) is a 9d Korean professional who has taken up residence in the Los Angeles area as of 2008)
Go seemed to progress in a lot more fits and starts than did chess (which, admittedly, probably had a lot more effort put into it). Prior to 2005 or so, Go programs were relatively weak and there were people working on them who were saying that they didn't really see a path forward.
Then people hit on using Monte Carlo which was the big step forward you show in your graphs. But then, that progress seemed to stall to the degree that various people were quoted in a Wired article a couple years ago about how they weren't sure what was going to happen.
