I think we could start looking at the last 5 editions as interesting. Yes, average speed is crude and dependent on many conditions and tactics, but when they start adding up like that over the trend line, it does make one raise the eyebrows a bit. Obviously not conclusive in any way, but...interesting. You don't see 4 races clumped together like that since the late 50's.
It's the kind of data which makes you want to ask more questions, not draw a conclusion.
Article: https://www.cyclistshub.com/milan-san-remo-statistics/