I agree with you that there really will never be a GOAT. Some are coronating Brady today but some one will come along one of these days and he'll be the next great one.
It's not that somebody might come along along and surpass Brady in rings and stats. It's that the idea of identifying a GOAT is, I dare say, frivolous. It's not done in other realms of human endeavor.
You're not going see any serious discussion of whether Archemedes, Newton, Einstein, or another esoteric choice is the GOAT physicist. Who's the greatest novelist? Is it ****ens or Melville or Joyce or fill in the blank? Who's the greatest painter? DaVinci or Van Gogh or Picasso or whoever is currently drawing the biggest money at auction?
While these other human endeavors are matters of considerable academic analysis and where one giant builds on or reacts to the accomplishments of predecessors, sports (and particularly football) are not so thoroughly considered. Further, there is not a lot of visual evidence of football performance prior to video tape. Such matters are left to institutional memory. You're not going to find many 80 year old former players/coaches/media with blogs arguing the merits of Graham or Unitas. Who?
Here's a thought. Transport Tom Brady back to the 1940's. That funny sidearm-ish throwing motion common in the day, as we see in the little film available from the era, was a byproduct of the fatter football with still some remant of the rugby ball. See "Slingin'" Sammy Baugh. The game was brutal. A QB had to be one tough ****. How would Brady do? Could he play two ways at defensive back, as Baugh did, and how might that affect his offensive performance? Could he punt as Baugh did?
You probably are not going to find many current commentators on the hunt for clicks comparing Brady to Otto Graham. First, they never saw Graham play. Second, the youth market has no interest in hearing about some guy from 70 years ago who's stats would laugh him out of any fantasy league.
Getting back to Starr, I believe there is a fair comparison to Brady. They were winners. Neither was regularly a statistical leader in any one season. Neither was a great athlete. Both played for among the great coaches of all time (another false GOAT topic), both subject to questions of being "system QBs". Starr was the beneficiary of the Power Sweep (which had many variations run off of it). Brady was the beneficiary of Belichick's innovation in the use of the slot receiver. We could make a similar comparison to Montana/Walsh in a transitional era of the way the game was played as illustrated in my previous post.
At the time of Starr's retirement, Graham as the so-called GOAT was already fading in comparison to Unitas, a Starr contemporary, throwing more often with bigger numbers but fewer rings than either Graham or Starr. Statistical distortions are nothing new. And we cannot underestimate the bias of recent memory.
I see no reason not to put Starr on par with Brady. It's almost like the answer to an SAT question. Starr is to Unitas what Brady is to Manning. Now, one might say that Brady throws the ball twice as often as Starr. That makes winning and losing more dependent on the QB than in Starr's era. Hmm. But Starr called his own plays. There are no field generals anymore in the true sense. QBs are passers, and increasingly valued as runners when things break down. Calling audibles is a vastly more restrictive control of the game than calling the play in the huddle with the full play book at the QBs disposal.
When you start thinking about differences in context in one era versus another, picking among potential GOATs is comparing apples to oranges. Going by stats is like asking, "If Newton was so damn smart, how come he didn't come up with General Relativity? Gravitational theory, calculus, meh. There are no women in "Moby ****"; that makes it just half a story."
There are no GOATs. There are the greats, and then there are the other guys. And the greats are great in different ways in different contexts in different times.