Miners will, however, have the option (and they have the option right now, too) to change the 21 million rule, and reward themselves whatever they want for each new block. No change to the official bitcoin client is required, because most mining is done by mining pools which can make the change to their own modified clients.
Bitcoin is the only deflationary currency I'm aware of, and is a central part of its success. If you want inflation, use any of the several hundred government currencies available. Or fork the block chain and make one that competes.