As wonderful as long term deals are, if Apple signed a long term deal with a supplier who didn't secure their supply chain properly, said supplier is going to either:
1/ Force apple to eat a price hike. Failing that, they can:
2/ Terminate their relationship with apple
3/ Go bankrupt trying to sell $2 for $1 (which leads us back to point 1)
That has happened before with GT Advanced Technologies over iPhone screens. Apple frontloaded GTAT's Arizona facility buildout, but the contract had no obligation to buy and GTAT's yields were horrible, so Apple didn't pay the last installment of the loan and GTAT by contract couldn't sell to anyone else.
iPhone 6 launched without the sapphire screen and GTAT went bust.
Apple has a bit of a storied history of being the reason suppliers go bankrupt. Their contracts are usually one sided (who is going to tell Apple no and turn down that business?), and Apple does not like to be pushed around. They use multiple sourcing and pit their suppliers against one another and aren't above bullying.
1/ Force apple to eat a price hike. Failing that, they can:
2/ Terminate their relationship with apple
3/ Go bankrupt trying to sell $2 for $1 (which leads us back to point 1)