I believe the better way to look at it when it comes to DoT clipping isn't the value of a DoT vs what you could do instead, but the value of a DoT's duration living out + filler vs refreshing a DoT. Otherwise you'd be making the same argument any time a DoT can be clipped enough that it'd be more potency than non-DoTs, which obviously isn't the proper way to DPS.

It'd be FT combo B4B + the remaining ticks of the old CT vs what a clipped B4B CT combo without old ticks would do, and I'm quite sure in this case refreshing early would lead to less DPS.