I actually don't think they're holding the club to ransom because they're not getting games at 18, more leaving because they know they won't get them at 19/20/21 either. They're just cutting to the chase and not wasting three years of their career on pointless naff loans/scratching their arses in the terrible u23s league. As someone said, not a single academy lad started in the PL for us for 6 years...and even that was a bit of a farce due to a mass injury crisis. Great education, yeah, but very little job prospects beyond that.
I always think of it like getting a scholarship at some massive company, which gives you access to all the best training the sector has to offer, but you know getting a promotion past 'junior' level is nigh on impossible given the quality of employees there (you've heard tales of some guy working as an intern for 18 months essentially unpaid getting increasingly frustrated etc), so you leave to an emerging company who has an exciting rep, though smaller, but one will step you up and trust you knowing you have that great education. You get a promotion as you take the job, you're a priority in a small team - you fully well know that you'll probably be able to progress quicker there and then go back to that big company as a senior member of staff 3/4 years down the line.
No problem of players at 18 thinking there good enough to move on get game time or start games.