No one can beat Father Time..undefeated!!..
Tom Brady: "Father Time? Yeah, I kicked his ass!"
Tom Brady is Father Time's father
Brady is obviously still great but he also obviously has gotten worse. His TD% has fallen 3 straight years just as his INT% has gone up. His completion percentage down those same 3 years, as is his QBR and his QB Rating, while his yards per game are down 4 straight years. Tom Brady is not the same player he was. Father time is taking its toll.
I think last season was bad, but I think that was due to the entire Pats being in dysfunction rather than anything else. Also, simplifying quarterback play into things like that is silly. That's pretty obviously not what TB is about.
Additionally, you're weirdly focusing on this 3 year period, rather than comparing it to the rest of his career. 2016 was his 2nd highest quarterback rating season in his career, 2017 his 5th highest and 2018 in the top half. The 2018 season is the only one of the last 3 where his INT% was in the top 10 for his career, while all 3 of the seasons you listed are in his top 10 for TD%. All of this is per pro-football-reference.
Hardly as if he's declined.