If Bitcoin becomes widespread enough that most people hold their savings in it, then it will greatly and permanently reduce liquidity in the financial system, lead to a decrease consumer spending and cause major ructions in the global economy.
This is bullshit Keynesian Economics. It's like saying that people spend only because they're aware of the inevitability of inflation. With no inflation they'd have no reason to spend. How dumb you have to be to believe that? Are you trying to make us believe that people would never buy a new car if they weren't inclined to do so by inflation, or that they would never buy food if they had vehicles that permanently increase their purchasing power?
Every time my sister makes a dumb purchase she defends it by saying her money won't be worth anything in the future so she may as well spend it now. I do not believe she is an isolated case, and her reasoning isn't wrong either. People, even those who don't know what QE is, intuitively have come to understand that their money will be FAR less valuable in the future. The only way to combat that is to invest but most people think of investing as difficult and risky, so their bias is to spend all of their available income. Bitcoin provides an incentive for people to save their money, the incentive being that their bitcoin won't lose value systematically. This incentive does not exist for fiat due to negative real interest rates and future pending QE. A world in which Bitcoin achieves mass adoption will clearly impact people decisions on what to buy, how much to buy, and at what price.
People will of course still buy the necessities such as food and a car but the psychological hurdle to clear before buying frivolous items will be raised. And of course, economic growth depends
exclusively on the sale of increasingly frivolous items.