I've answered this before.
The sooner we go over- the sooner we will have to reset.
I am thinking our chances of winning in 3, 4 or 5 years will be better than 2020's chances, so staying under now, buys one more year before the next reset- in theory.
I realize the whole system could change next year, but it appears the Sox are thinking they want to stay under, this winter. I'm guessing their long term plans out weigh the short term ones, at this moment in time.
I hoping that changes, next winter. I'm pretty confident it will.