Shorting bonds can be expensive over the very long term. However, the lower the rates the cheaper it is. You're "paying" the coupons to the bond holders. Being long a 2x inverse 10y treasury ETF @ 2.5% interest rate plus ETF fees can cost approx. 6.5% annualized. That's a steep hurdle to overcome.
Your potential gain comes from an abrupt rise in rates. Over time, the losses on coupons can outweigh potential gains on "slow" rate increases.
