This question already has an answer here:
Can anyone with enough computing power, take over the network ? This is following from a previous comment where I was asking :
... how can that be possible (to control the network) if mining tend asintotically to 21M ( alias the end of mining ) ? The only control is on early production of coins, not on the allover crypto-economy IMO.
where I'd been answered like the following :
Blocks do give miners coins, but that's not their purpose. Their purpose is to keep an ordered list of transactions. By using 51% of the mining power, they can prevent transactions from getting on that list indefinitely.
What do you say ?