The problem isn't the price going up, it is the wild back and forth swings as leveraged traders enter and exit trades that is hampering Bitcoin being used as an actual currency. When the Russian rubble started to drop in the last few weeks people were scrambling to buy any hard asset they could get their hands on to preserve purchasing power. So the same concept applies to Bitcoin. Stability is necessary for bitcoin to flourish as a means of exchange, otherwise all we have is a fancy new casino.
if everyone wants the price of bitcoin to go up, y don't we create a system such that no one can sell lower than the last price?
Margin trading doesn't create volatility. The underlying creates volatility. No one knows what bitcoins should be worth. That's the primary issue. It really doesn't matter if it's one dude buying and selling Bitcoin with 10x leverage or 10 people buying and selling Bitcoin with just cash. It's the same.
We need MORE traders, leveraged or not. The spreads are what makes BTC unusable for daily transactions, not volatility. Imagine you go to a store that lets you pay with BTC with a spot price exchange....
For the buyer, he doesn't want to pay in an equivalent amount of BTC because he's going to end up paying the Bid which might be .5 to 1% greater than the Ask price.
In that situation why would I pay in bitcoins when I can pull out my credit card and get 2% cash rewards?