I have purchased seven different TC units over the past twenty-five years, ALL of them used. The modern Nikon AF-S TC units have no significant moving parts, and are pretty sturdy. The thing is, prices on used TC units vary widely--I bought one TC-14e for $105, another for $179, even though the retail price on them is near $400,and both were like new. TC units "sometimes" are priced really below market price, if the seller is motivated or uninformed. It pays to look around the web in the Used section of smaller web-based stores for good deals.
Like Mr. Logic, I would not really expect Nikon to be updating its 1.4x and 1.7x TC units to aspherical design next year, but who knows--it is a possibility. If they do, that alone will , I think, lower the used market values on the older,non-aspherical style TC units by a bit.
My gut feeling is that the new TC-20e-III is designed to be optimal with the brand-new, announced-this-week,updated version of Nikon's first generation 300mm f/2.8 VR-G super-telephoto lens.