But that's exactly the problem. With higher interest rates, those deposits will be looking for a higher deposit rate. With their assets tied up in low-paying long-term bonds, SVB will not be able to pay that higher rate.
It would only work out "eventually", if the depositors would accept a below-market rate until the bonds matured.
EDIT: This is indeed a solvency issue, not a liquidity issue, as also pointed out below.