The main issue to me is northeastern passengers getting less value out of Northern-only tickets; if they were valid on some TPE services, even if it were just between Thornaby and York (which likely don't have sufficient capacity), it would help.
Some working-age people outside the southeast do have railcards, but I know it's for brevity's sake rather than saying "fit non-veteran working-age people travelling alone outside the boundaries of the Network and other regional railcards".
That's an interesting point between Thornaby and York.
I get that service quite often and usually pretty quiet (which makes it so annoying that they seem incapable of coming up with a reasonable weekday fare to Whitby).
In terms of the other railcards, I admit I have one of the regional railcards, but its application is highly limited, unlike the Network card !