Year 4 and having shown so signs of development? Well, I guess you could argue he has shown SOME signs of development (better pass %, better turnover %?) With Tomlin's obsession towards pick offs, it's impossible to say.
From what I understand from the tons of breakdowns from various analysts... Fields has improved in a bunch of things. They worked with his mechanics and his outside passing is far more accurate.
The big problem he is still lagging on is reading over the middle stuff and secondary routes.. and that is almost universal for younger qbs in a new system..
He absolutely still has a ton of potential
And guys can develop in any length of time... its just a matter that after a certain point teams give up and move on to the next thing to keep fans engaged...