So, according to OP and some of you guys, if BTC is massively adopted under this "accept BTC and sells it for USD" system, the price will collapse. Suppose more and more companies start accepting it, the price will go down more and more, lets say 10 USD each. Then, any company willing to do a 210 mln USD transaction would be able to buy all existing BTC (because in your guys logic the part where the buyer must buy the bitcoins so the company may dump it is simply forgotten
 Yeah, the part where most bitcoins anyone holds now were bought and not mined - they are not the "first owners", that is what I mean - is ignored.). Pratically all coins spent and later "dumped" will have to be bought for USD from someone who did the same. The amount of "first spenders" relatively to the rest will become less and less relevant in terms of volume to a point where all coins must be bought for USD before being "dumped", which will annulate this effect (when I say USD it can be any other currency), unless people start holding BTC (the amount that can be mined is limited, so at a certain point you either bought them or earned them by other means than mining), which will drive price UP, and not down. The conclusion that mass adoption followed by currency convertion leads to a collapse in its value leads to this absurd conclusion: if increasing adoption reduces price, then we will certainly arrive to a point where an individual or a group can buy all of them. But how can that happen in a mass adoption environment? Who would sell "the last BTC"?