this is so far from the truth it's not funny.
in a prevous life i had exposure to NRL contract set ups, and it's all about PLAYING first grade rather than if it's bench vs starting which sets up how much you get paid.
a starting prop can make just as much as an interchange prop, they both play similar minutes and the exact same role.
part of the reason you get more million $ backs than forwards is that you need less backs than forwards for a game (gvien that a back is expected to play out the 80 minutes vs a forward who may play 40 or 60 minutes..)
but in short, there is nothing wrong with being a super sub..
craig wing, dean widders and leeson ah moo have made themselves household names doing it..
I was basing the statement on looking at the teams across the competition. The quality across the board is not too strong as far as interchange goes and that appears to be the general trend. Maybe I'm generalising too much but that's what it looks like to me.
I agree with your point about props but I also think for that reason it's harder to have multiple quality middle forwards playing together, because they all deserve more money. If there is someone that has a higher paid player that is able to be an impact player off the bench, there is usually other someone else in the starter job for less and will either continue to do that or will be in demand when they are next off contract, basically like Ah Mau is now. Ah Mau has been underrated for a few years which has allowed us to keep him for less, now he will be rewarded. We still hope he stays around for less and have offered something competitive but probably will be well paid by a team willing to start him.
As for the other examples I would say having a player of Craig Wing's (who ended up starting a lot anyway) quality on the bench would be pretty rare these days. If there was a player like that around now then chances are they will either end up starting (in a 'spine' position) or moving elsewhere. Connor Watson was compared to Wing and he ended up leaving the Roosters. We have had similar examples past few years like Segeyaro, Luke, Cook who would have been more suited to playing off the bench but ended up starters and rising in prominence.
I don't remember Widders' career that much but if he was any good then coaches out there would be trying to turn him into a starter these days. If that worked out he could probably be making more elsewhere as a starter and/or would have been hurting his chances for rep honours by doing so. Dave Taylor was an example of this (when he was decent). Another example would be Sims, who is more suited to the bench but now has come back into Origin calculations as a starter. He pretty much won't move back unless he makes a sacrifice to do so.