Pump failure isn't the big issue, although it also is a point of easy catastrophe. The problem is mismatch. Let's say you buy two identical pumps that move 3200 gallons per hour. Let's assume for a second that they engineered by some divine being and there's literally no variation in that number (there will be even among two identical pumps). The second one of them slows down due to age, gunk, mechanical wear, whatever, let's say it now moves 1% less water. Now your 75 gallon sump is either gaining or losing it's entire volume of salt water every day. What will correct that? You can't just drain it or you'll drain your tank. This will require intervention every single day... Now imagine the difference is 10% or more.
Other considerations... How will you correct for evaporation without a "bottom"... You won't have a single chamber whose level depends entirely on how much water is in the system, they will all vary based on the effective relative rates of the pump and the last time you intervened. You'll have to handle evaporation top off manually based on measuring the systems salinity.
The reason everyone is reacting like they are is because you are introducing so so so many points of failure into something where even minor failures result in loss of life, money and often damage to your house.
