[VMG]

Individual clients don't have to use the same client that miners use.

If 51% of miners start using an inflationary version of Bitcoin "Bitcoin-QE", but the clients (exchanges, merchants, wallet providers, etc) don't approve, they won't update the client and use the partition of the network that is secured by the remaining 49% of miners. People wouldn't want to accept the Bitcoin-QE mined on the inflationary branch and the investment wouldn't pay off.

Of course Bitcoin-QE miners could start an attack on Bitcoin in other ways, but that also woudn't be necessarily economical.

It's very hard to tell what 51% will do, but it is clear that you don't have full control of the currency.