That's a valid point, although they've spent a lot of money on the whole train in recent times. They doubled the size of it. That, and the acquisition of the F-units, would've been unimaginable a few years ago. They're operating the train a lot more frequently than they did in the '90s and early 2000s.
On the other hand, if the F-units really do need more work than they're worth, it would make sense to power the business train with a couple of freight units that are already PTC-equipped. I guess it depends on how important the streamlined look of the train is (they never cared about that in the past, FWIW) and how much work the F-units really need.
Pan Am Railways — Boston & Maine/Maine Central — Delaware & Hudson
Central Maine & Quebec/Montreal, Maine & Atlantic/Bangor & Aroostook
Providence & Worcester — New England — GE Locomotives