You seem pretty sure of that breakdown, and I'm not saying there's no significant chance of a steep decline, but to me, Betts will earn his 10 year contract in his first 6-7 years of the deal. Yes, it would suck, if he was horrible the last 3-4 years, but I'd still sign they guy longterm.
I think the plan will be to hover around the tgax line until July (No JBJ & Leon, at least at their arb rates), then we'll decide to keep Betts and make a playoff push- perhaps going over the tax line to do it, or trade Betts and maybe others to improve the rebuild chances and quicken the turn around. (Personally, I'd still make him a hefty offer as a FA- maybe $320/10.)