What I've read is that, the SPR was designed for a 25 year lifetime and 5 purge/fill cycles. It's now at 40 years and dozens of purge/fills, so it's way out of it's design parameters.

The problem is that as you pump oil and water in and out of it, in the intervening layer between the salt water and oil there forms a water/oil/impurities sludge which is useless (it ruins the pumping equipment). The thing is, they have no way of measuring it, nobody knows how much good oil is left and how much is sludge (or if they do, none of that information is public). And the closer you get to the sludge, the harder the good oil is to extract, slowing down the extraction pace.