Your big unstated assumption is that any intelligence will always consume the maximum resources it can and cause the maximally possible transformation of any habitat. Normally, when physicists look for these kinds of large-structure signs, the implicit assumption is that it's sufficient for one civilization to behave this way, but you seem to assert they all must be like that.
Any of the hypotheses we discuss in this thread (whether they are over-confidently stated as facts or marked as speculative) are inevitably deficient due to data derived from a sample size of one.
