I'd get so incredibly bored if I had been unbeaten for 5 seasons, once you've got that sort of team where's the challenge? In my eyes there's no point in playing a game if it's not challenging.
I've got a pretty damn good team with Man City, in the first season I won everything, so in the second I lowered the speed of my players and upped the speed of the computers and raised the difficulty. It can get pretty damn frustrating but it's so much more rewarding when I win rather than breezing through every match.
Eh. For me, it's impossible to actually have a
real challenge against the AI (i.e. without gimping myself or powering them up) and even if I could, the AI cheat so rampantly (ignore Strength, Agility & Reactions stats 100%

) that it's just not that fun to have a close game with them, because it feels nothing like a close game against a person.
I use my games against the AI to try out new strategies, tactics, formations, playstyles, whatever; my real enjoyment comes from playing same-system against my friends, the CM is just for laughs & to see the player growth.
I never realized some of those players got that high.
So, for this, it's a combination of a few things.
First, I have a theory about how FIFA player growth works: Players will continue to grow as long as they're in good form and are playing games- the 'potential' you see on like fifafanatic is the max they will reach on an AI team. With a player controling them, it becomes possible for the player to grow much more.
I believe that the growth system looks something like this:
f=average player form this month m=minutes played this month a=player age g=growth
So, as you can see, if that's kind of the way it works, there is no real limit on the upper end of player growth. I think the system has some variance, because obviously players with high potential grow faster, but I think that's down to this: There is a multiplier on the value for M that accounts for the player's potential; this allows for players to grow faster up to their potential, but then the growth slows down markedly (which has been my experience) which helps low overall but high potential players grow faster.
So, short version- since my players are all in good form and play a lot of minutes, they quite often grow beyond what they would on an AI team.
Did you start before or after the January update?
I started about a week after the game came out, so before
I have Bale, Shaqiri, and Bittencourt. Might sell Bittencourt this season because when I bring him on as a sub when Gotze or Eriksen gets tired, he just kind of does nothing or gets muscled in the middle of the pitch. I love his 5* week foot though.
Courtois is at a 91 right now. His stats are ridiculous. All keeping stats are dark green besides kicking and that is 82. Mental. I wonder what Neuer looks like when he is fully maxed out
Bittencourt is really good, but he's a niche player. He really only excels as a CAM; he's not good enough defensively to play a CM role, but his pace & ability on the ball make him really good behind a striker (or 2) so if you play a system that uses a CAM, I think he's worth keeping. Never been too enamored with Eriksen myself, he's just not a big goal threat which for me is important in a CAM...
I've heard that Courtois gets really good, but I just like the way Ter Stegen plays too much. He's so strong & aggressive, he owns the box like no other keeper I've found.