The maturity value of a bond/mbs is not really the true value at a given time between it being issued and maturing. You can treat it that way as a person holding the bond if you pinky-promise to yourself to not sell until maturity, but since banks need to periodically sell these to let customers get their deposits, that fiction doesn’t work for them.
These things trade on the open market and adjust to interest rate changes. What really happened is that they bought a bunch of securities backed by depositors that lost value at mark-to-market. This is poor risk management, not some unfortunate unavoidable issue caused by a bank run. They are insolvent at market prices - illiquidity would be more like they have a bunch of contracts or snowflake assets (like buildings, or a security that doesn’t trade on the open market).
As a depositor, saying “in 10 years you will get the full value of your deposit back” is bullshit: you could give me the actual reduced value now, and I could invest it in the same kind of instrument, and in 10 years I’d also have the full value back - but I could do other things with it too, which may be preferable considering it’s my deposit that I may need to spend now rather than in 10 years.
The fear is that many other banks are in the same position, that they are technically underwater and vulnerable to runs because the true value of their deposit-backed assets have decreased due to interest rate increases, but that SVB was affected first because their customer base of VC-backed businesses just happened to have their withdrawal:deposit ratio increase the most.