I wasn't sure if we were metaphorically talking about the foetus brain in question or a hypothetical human that's fully grown in an isolation tank. If we were talking about the former, we seem to have a fundamentally different set of ethics. With your clarification I assume we're talking about the latter, in which case I agree with you.
Saying that an undeveloped foetus brain isn't thinking because it hasn't received sensory stimuli is a different argument than saying that a fully grown brain can't think because it hasn't received sensory stimuli.
This seems significant:
http://www.theguardian.com/science/2015/aug/18/first-almost-fully-formed-human-brain-grown-in-lab-researchers-claim