Tommy ring, Collins, Kay, Young, Ball, andy king, Joe parkinson
Many killed in WW1 ; who knows ?I don't think players being forced to give up the game because of injury fits the OP criteria of hanging up their boots "too early".
It has to be players who just quit for one reason or another whilst still capable of playing.
The best example I can think off is Peter Knowles of Wolves and England.
He was still in his prime when he announced one day he was quitting the game because he had found God and was taking up preaching![]()
<blockquote class="twitter-tweet" lang="en"><p>“<a href="https://twitter.com/Everton">@Everton</a>: RM: "Steven [Naismith] has concussion and couldn't remember scoring. I've told him he scored from 40 yards." <a href="https://twitter.com/hashtag/EFC?src=hash">#EFC</a>” not funny</p>— Taylor Twellman (@TaylorTwellman) <a href="">February 16, 2014</a></blockquote>
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George Best : even when he went to the States
Twellman had issues with concussions while playing, and has become an advocate for proper testing of players if they're suspected of having one.