A few brain function tests, when Terri was still alive, might have supported your point somewhat.
But, alas, the tests were never run.
They might have if the Schindlers had been willing to stipulate that they would drop their case and allow Terri to die if the tests proved Terri was in persistent vegatative state. Since they wouldnt, why bother? They wouldnt served any purpose? And what if there was still question following testing?