I am confused. Why the assumption that Asimov *ever* thought the laws weren't lacking?
I admit, I haven't read everything he wrote, and what I did read was a long time ago, but my impression is that he came up with the laws specifically to tear them into shreds, by writing stories where they are subverted, ineffectual, or otherwise limited in effectiveness.
I see them as a literary device and a reasonable assumption to build stories from, not a proposed great way to solve everything ever. And yet I continually see them interpreted as Asimov's idea of what perfect robots would be like. Why? He examined how they could not serve their purpose in his own stories.
EDIT: according to Wikipedia, "Runaround", the very first story to explicitly feature the Three Laws of Robotics, is a story *about* how a robot nopes pretty hard on the laws due to how expensive the robot is. The Third Law takes precedence over the Second Law in the story, which is a clear violation of how the Laws work.
So the exact moment Asimov formulated specific Three Laws of Robotics, he *immediately* set out showing how they would be rearranged in specific circumstances.
EDIT 2: and - again, as per Wikipedia - the previous story to imply the existence of the Laws, "First Law", is about how a robot directly violates the First Law to protect its "offspring". The story, according to Asimov himself, is a parody, which blunts the point a bit, but still - even as a joke, he was already playing with what violating the Laws would entail.