Well,
Same is told about gold and silver. I believe without using dirty tricks the price would be much higher. And NOT on a limited extend. Every form of manipulation, even for a short period, should be banned.
EDIT Price of gold is manipulated since Nixon closed the gold window. That was not yesterday :-). So they could bring down prices for a long, long time...... Way too long
You have a good point here, but here is the key thing you're missing:
"the price would be much higher"
Right there you're measuring against something else, presumably dollars. In a gold only economy where people exchanged only physical gold then 1 oz of gold would get you a certain amount of goods/services and that wouldn't be affected by external systems, at least not in a way that drove down the purchasing power of the gold, because there would only be a certain quantity of physical gold each participant had.
The same would apply to bitcoins. Once you have an economy that largely transacts bitcoins you won't be able to drive down the purchasing power of bitcoins with external systems, no matter what they do, because there are only a certain quantity of
actual bitcoins each participant can have.