With regards to the Runningback Discussion:
I don't think it's quite that simple. Like, different roles in each sport have different longevity curves, but I can't think of another example at least in the four major sports where one position's age/relevancy curve related to value is so sharp and short as runningbacks and it creates a unique inefficiency as that knowledge is now common, and it has suppressed draft value, contract value, and team building value. Runningbacks at their peak provide massive value at well below value salary but the curve falls off super quickly and they can't logically cash in on that like pretty much every position can.
Runningbacks have an issue that the things that have the most value to teams, that they'd hypothetically pay the most for on a perpetual open market, are things that severely limit how long the runningback can play. Yeah, a RB whose primary job is a third down back can have a very long career, but third down backs don't generally get paid very well. Darren Sproles was a 3-4m a year guy and he lasted 14 years because he never took more than a hundred carries in a year. Dalvin Cook has five hundred more rushing attempts in his six year career - only one of which he played more than 14 games in - than Darren Sproles has over 14 years. That's 500 times being slammed to the ground by a 300+ lineman or being hit by a 250 pound LB flying around the field.
In terms of WeightedAV, Dalvin Cook is the 8th most valuable player from his draft class behind Patrick Mahomes, T.J. Watt, Alvin Kamara, Myles Garrett, Ryan Ramczyk, Christian McCaffrey, and Deshaun Watson. All the non-RBs in that group are slated to make 17m plus in the upcoming years - even the lineman. Alvin Karmara, Dalvin Cook, and Christian McCaffrey all made very solid money but nowhere near what other players producing similar value will make unless they beat the longevity odds - and given how those three have missed notable time, I wouldn't expect that to happen). Four of the Top 10 players in that class are RBs (you need to go down to #35 to get to the first DT - Dalvin Tomlinson - who got a 4/57 deal in Free Agency).
RBs have a "choice"* in that they can provide maximum value to their team, get hit 200-400 times a year by the biggest, strongest players in the sport, and rapidly decline before they reap the true rewards of a big contract or be one part of a platoon and play longer but earn a small fractional amount of cash. The difference, to me, between an RB and a DT is that an RBs job by team design, game necessity, is designed to grind them into a useless pulp more quickly than DT. I don't know if there is a clear solution to this to be honest - like saying a team needs to pay RBs differently despite obvious quick declines in productivity isn't realistic, but it is deeply inequitable and it totally makes sense why RBs are upset about this because the system is actively disadvantaging their position group specifically because if you are really good, the team is going to run you out there for a ton of carries and destroy your future value. It sucks. I am guessing we'll see a lot of college runningbacks either leave college as early as possible to maximize their chance of getting to free agency with tread on the tires or outright change their position.
*Not really, a rookie runningback is going to have no business telling a team they should load manage his carries and throw him a ton out of the backfield to lengthen his career