Depends on your definition of ruined career and what a player is really playing for. You stick with me Rodney and next year we be millionaires!
They're professional footballers, the key word being 'professional' and like,most people who aren't born into money, part of their motivation is to earn a good living.
Taking Slew as an example, one could argue that if he'd stayed with us he would have played more and, if he'd done well, might have got a move to a bigger club as a more senior player, like Harry did.
On the other hand, he might not have been able to displace the Evans/Cresswell partnership, been a bit part player and drifted out of the game, like most of his FA Youth Cup contemporaries. Had he done so, he'd never get the chance to earn decent money or have a crack at a high level. The fact Slew didn't make it isn't his agents' fault. For a striker who rarely scores he's actually played quite a bit of senior football.
I'm sure there will be the odd rogue still out there but I don't think it's like the days when agents first started out when there were some real wrong 'uns about.
Clubs, particularly those that have to sell players, like to give the impression that it's the agents that cause them all the problems but I think in most cases, it's the players who actually want to move and tell their agents to find them a bigger club who'll pay them more and get them closer to the top level.