I would agree that it's a cop out.Seems kinda like a cop out. Which is no shame, every shounen has one.
I don't think Kubo had all the details concerning Isshin in mind when writing the Arrancar arc or we would have at least had a moment where Rangiku and/or Hitsugaya see Isshin or note Ichigo's resemblance to someone they know (which could be mistaken for Kaien to hide the fact they actually mean Isshin).
It's the same as how Rukia would have obviously spotted the similarity between Ichigo and Kaien, but there's nothing to indicate that in Chapter one to explain her otherwise inexplicable behaviour (telling Ichigo anything instead of just memory replacing him on sight).