It's ALL conjecture until a contract is signed. You all know how NFL high profile negotiations work... two parties approach the table with opposing goals in a volatile profession resulting in weeks of unrealistic offers and demands from both sides until they eventually settle somewhere in the middle where both hopefully feel they have successfully limited their risk exposure. (Whew!)
TW, your argument appears valid. Given football’s inherent violence, the physical toll it inflicts on its players, and their shrinking career spans (currently 3 years-ish), it appears NFL players are the professional athletes most in need of guaranteed contracts.
Yet it's that exact same reasoning why owners are resistant to giving guaranteed contracts... and I can’t blame them if they want their team to remain competitive. They have to deal with A SALARY CAP, players are one play away from major injury, teams have much larger rosters to pay than other pro sports, there already is a large disparity between the veterans and young players, the NFL offers every benefit that is provided by all other pro leagues plus several more - severance pay, long term care insurance, the Former Player Life Improvement Plan, and cognitive disability benefits. The only result I see of mandatory guaranteed contracts is shorter contracts because of risk of loss of competitiveness and loss of benefits.
In reality, contracts are really just word salads and depending how written (e.g. injury guarantees, opt-outs, % of cap) a totally guaranteed contract may be less desirable than one with bonuses and 50% non-guaranteed depending on either sides aversion to risk. (Complicated, right?) What's interesting to me is when Flacco signed his contract of $20M/yr after Rodgers of $22M/yr and that resulted in a market of not much pay disparity between an average QB and an elite one. The recent contracts have widened that gap.
If re-negotiated this year Rodgers' salary structure will be modeled after the recent ones: around five years $157.5 mil total ($31.5 mil/yr), $96 mil fully guaranteed, with $100 mil 3rd year cash, $55 mil 1st year cash. An opt-out makes no sense for the Packers, and salary based on % of cap is too risky for Rodgers in case of another rollback.