You don't think the Primary Dealers are guaranteed a profit, do you?
If they could not profit on the trade, they would not trade.
They profit on every trade?
They are not in the business of buying Treasuries to hold or to lose money.
How many Treasuries do they hold in inventory?
If ONE major buyer steps out of the market (like the Fed with its 40% of the aggregate demand)the primaries are stuck holding bonds.
If you want to play in the big leagues, sometimes you get stuck holding bonds.
The Fed is buying up bonds to keep interest rates low not for fun and excitement.
Is it working?
If interest rates continue to increase, bond holders will see continued depreciation in portfolio value unless held to maturity.
I'd hate to borrow overnight to finance long term Treasuries if rates continue to rise.
Per your graph, if the Fed is only slowing the rate of increase in 10 yr rates, then The Comedian needs to post that great photo of the mom and little girl in the roller coaster. If the Fed can’t control interest rates by buying “excess” treasuries from the Primaries, then the rope is slipping through the fingers. I hope you are wrong but I fear you are correct. The Fed, ultimately, cannot control interest rates in a global market without folks like me living in dirt floor huts... Gloom and doom prevails...