Well, I think the "sign JD and trade Bryce" group would say an OF of JD / Taylor / Eaton is probably as good offensively in 2018 as one of Harper / Taylor / Eaton. JD has topped Bryce in wOBA 3 out of the past 4 years, including the past 2. What kills him is the defense, which lowers his fWAR, but you could say Eaton is replacing Bryce offensively and Martinez is replacing Werth /Lind / Kendrick etc..., making the defense better than 2017. The team acquiring him could offer him a QO if they could not extend, so you might get good prospects back, especially if they are a year + away. It starts to look less like you are weakening 2018 and instead looks like you are improving 2018 and positioning better in the long term.
How does it better position for the future to give major long-term money to somebody in their 30s in JD versus Bryce at 24? That is only if you assume JD wants to come here and Bryce is 100% gone. The JD contract could become very, very bad in a few years. It almost certainly will be bad after 3 or 4 years.
