Most of the time there is in fact a consensus on who the best players are at least the 3 or 4 best. Sure there are years where a couple of players may be 5th and it is true that the playoffs may help clarify that. I never said it was scientific and you can nitpick a season here or there, but you know what you can't nit pick, 90% of the seasons, you know when Mikan, Bill, Wilt, Kareem, Bird, Magic, Jordan, Hakeem, Duncan, Shaq, Kobe, Lebron, Curry are winning multiple titles.
And I'd ask everyone to look at the last 10 Finals or so (you can go back further, it still generally holds true) and compare the stats of the best player on the champion verse the stats of the best player on the runner-up. Pay special attention to the runner-ups stats when that guy is Lebron (a clear top 5 player), verse basically anyone else. You can tell who the top 5 guys are on those final 2 teams. There is a huge stat discrepancy nearly across the board (except Lebron and 1 Curry year). I'd argue quite simply that is because of the clear talent gap between a special player verse merely a great one. There is a reason guys like Jokic dominate, while a guy like Butler is arguably out performed by Bam. The top 5 guys can do it night in and night out. They don't have the huge spikes in performance. That is what sets them apart and makes them a top 5 player. A guy capable of putting a team on his back and carrying them regardless or how anyone else plays.
I agree with you, mostly, but I would point out that the bolded is verging on tautology, which is what people are honing in on re: your comment - if we retroactively say that the best players are the players that performed the best during a given season (which is reasonable), then - as greenbb13 pointed out - you're essentially saying "the best players are the best players because they win, and the best players were the best players because we won".
So while this is true, we do have to balance this logical fact (how can you be the best if you don't win, after all) with the notion that 'past performance is no guarantee of future results' - which is particularly pertinent to athletes, who tend to curdle like milk. We can say that these guys have been better than Tatum thus far, and I don't think anyone disagrees, but to say that Tatum will always be worse than those guys (however we want to define better/worse) because he's always been worse than those guys thus far is self-evidently incorrect. That's what people are reacting to.