The reason why Michael Parkinson is so oddly skeptical is because
While Pipes is possessing him, he's making him get more and more opposed to his own [Pipes's] existence, similar to his making the supernatural occurrences appear to be a hoax perpetuated by the Early family.- This also adds up to Parkinson's actions as Pipes seems to take control of the broadcast at the climax. Parkinson is still smiling and laughing at the calls and reactions to the stories coming from the phone center. If Dr. Pasco had heard any of them, she may have put two and two together and tried to cut the broadcast. Parkinson, under control of Pipes, shifted attention back to the house.
Two of the victims actually survived and escaped the clutches of Mr. Pipes
Not an official canon thing but in keeping with the "immersion" factor and the continued existence of Sarah Greene on the "telly", it would make sense and perhaps provide a bit of hope. In the same vein, the written sequel story is written from Pipe's perspective as he possessed Stephen Volk, which is admitted as such in the end of the tale. Perhaps Sarah Greene was a powerful Druid priestess in a past life and has the power to destroy Pipes and Pipes has been reeling from not being able to defeat or claim her. Every time she's near, Pipes flees. A possible sequel could be about the team returning with a coven of Druids to purge the home and airwaves of Pipes once and for all (or at least contain him like the Eaters of Light in Doctor Who).