You didn't really account for slow down as you projected the same totals as this year. His next 2 seasons could easily be less than 50 goals. Also if he loses even a little speed it will that much harder to keep up with the play and get into good shot spots, have stamina and what not. I am not saying he is suddenly going to become a 20 goal or something, but less than 50 is still very likely and that doesn't even count any injuries. Ovy has played a harder style than most star players and when the wall hits slow down wise it could be much bigger decline than most would assume for him.
They could be less than 50 sure, but they could also be more. He was on pace for 57 goals this year so what if we see this instead:
55-48-43-40-35-30 ? Decline from previous year every year including the first and still 40 goal average.
There is nothing right now that leads me to think Ovechkin will decline by the time next season starts apart from the magic number 35. As long as he stays healthy, I don't think the decline will be as steep as you think.
Of course, if big injuries happen this could change, but until they do I'm not factoring them in. Ovechkin has been a tank his whole career (
played 1152 of a possible 1183 games or 97.4% of the games) and I don't think this will change for a few years still.