i dont think your wrong, i just think its more nuanced then that. to me, its not the contract its the age of the player. the relationship between contract number and age is spurious (a mathematical relationship in which two events are associated but not causally related, due to either coincidence or the presence of an unseen factor). the unsen factor is age. there is nothing about a contract number that is important. for instance, hypothetically if goff signed a 1 year deal and then signed a long term deal on his "third" contract. all things being equal, the numbers would be about the same. age is important because of its relationship to athleticism. in theory a second contract is signing a player in his prime.
further, ill accept that 15 percent of the cap is around the base for a starting qb in his early to mid 20s. however all starting qbs arent all the same. so while the contract may still take up a large percentage of the cap, better players will still in general get paid better. for example, if you look at the Contract Average Cap Percent Rankings Rodgers and Wilson are numbers 1 and 2. ill bet that mahomes will exceed the 15 percent. long story short, imo its not as formulaic as some suggest, players still get paid according to their perceived value within a rough hierarchy.