Expected lifespan estimate for SD card used in read only mode

It's quite likely - SD cards are delicate & fail easily.

I could find you the MTTF figures for your particular card, but it wouldn't really tell you anything except what it says - mean time to failure, the average life-expectancy of that specific model, not the life-span of any individual card.

I used to get through hundreds, if not thousands*, of SD cards for work. They would be write few, read many - so they'd have new data put on, which would then be inserted into a read-only machine for several months/years at a time, then cycled round again with new data.
Part of my job was to check the machines, repair/replace as necessary, throw away the failed SD cards & replace them - so I've seen one heck of a lot over 10 years of operation to form this opinion.

Some will rock on for years without a glitch. Some will start to fail in a few weeks. There's no way to tell which is going to do which.

The 'fix' is just to bin it & get another. It's pointless trying to fight them once they start to fail.
They're cheap, they're disposable.
They should never be used to store the only copy of any important information.

*I worked it out roughly - at any one time I would have just under 2,000 cards in the field, in constant use.