There are a few reasons why this year would be the best year to make a huge run at an elite asset like Scherzer:
1) The Red Sox are already right at or just over the luxury tax level. It doesn't matter how much you exceed the cap, it counts as a "year over the luxury tax level". Now, they have to pay an overage based on how far over the level (189 mil maybe?) they go, but the penalty is minor because they haven't been over that level in several years. Going over the luxury tax level in consecutive years makes the penalty escalate in terms of percentage, but given that the Red Sox have salaries coming off the books for 2016 (Napoli, Victorino), this might be the year to add the big contract.
2) Scherzer, who turned down the QO, would cost the Red Sox a pick, but since they've already signed a few guys with QO attached in Hanley and Panda, they'd lose a 4th round pick if they sign Scherzer. That's not much of a penalty to pay for this kind of addition.
3) He's the best pitcher likely to enter the free agent market for the next 2 seasons. Free agents after 2015 include Price, Iwakuma, Zimmerman, Norris, Latos, Kennedy, Fister, Porcello and Shark. I would rather have Scherzer than any of those guys, and the prices for the pitchers listed are only going to get more expensive. Scherzer at 7/180 might look like a good deal by this time in 2015.
4) He's the best pitcher the Red Sox can acquire. The guys who project to be better than Scherzer over the next 3-4 seasons aren't available in trade, and if they were, it would likely take packages led by the Red Sox top young assets: Betts, Swihart, Bogaerts, Owens, etc. Scherzer will only cost money, and while the contract might look bad if Scherzer ages poorly by the end of the deal, that might be the price of trying to win in the short term.