If I was travelling alone, I would happily move to a seat that was either the same as, or better than, the one I'd booked. But I wouldn't move to a worse seat. And if I was travelling with my partner I wouldn't move seats just so someone else could sit with their own partner instead, baby or no baby.
The bloke in the article had paid extra for his seat because it was a seat with more leg-room, so if he moved, he'd have wasted money on paying for something he couldn't use. Of course he shouldn't have moved. It was a 10-hour flight. Why should he have to be uncomfortable for ten hours when he'd paid extra to avoid that exact thing?!
Also, it's not like you cannot possibly 'help with the baby' unless you're sitting next to someone. If the mum wants to have a kip for a couple of hours while the dad holds/feeds/entertains the baby, she can ... just get up and hand him the baby?
Basically, in this man's case I'd have said no to the passenger and then, when asked again by the steward I'd have suggested that my partner and I would be happy to move if they upgraded us to business class, but otherwise absolutely not.