It was a very sensible policy from GTR. Consider someone doing a long distance journey like Exeter to Stevenage, where GTR is the connecting service from London to Stevenage.
If the train from Exeter to Paddington arrives on time, it is highly likely that a passenger will arrive at St Pancras (or Farringdon if routed that way) early enough to catch an earlier connecting train, as the connection time between Paddington and St Pancras is generous.
Is it really the intention of the industry to keep people hanging around on them platforms at St Pancras or Farringdon until the booked train? The platforms at those stations are busy enough as they are, without people hanging around for booked trains. Wouldn’t the right thing be to allow passengers to travel on the next available service? GTR don’t even do proper seat reservations.
In the other direction, someone wanting to take an earlier connecting train from Stevenage to London should be allowed, the main thing is booked service they should be on is train between Paddington and Exeter.
To be clear, I’m not arguing for reduced interchange times, many think they’re already too tight.