Soft starter for centrifugal pump
I have slight experience with residential well-pumps, pool pumps, and pressure booster pumps. None of them use soft start. Those I have dealt with are in the 3HP and less range. Single phase. Service life is OK. I think the centrifugal switch on the motor or the motor capacitors would be the first thing to fail. Maybe motor bearings for ODP motors in dirty environments. If you are dealing with 3 phase motors, I would think that you will get excellent service life without slow start. So I don't think it is advantageous. In a dirty/dusty environment, consider totally enclosed motors (TEFC or TENV).
Some types of single-phase motors do not have a centrifugal switch (permanent split capacitor motors). So a permanent split capacitor motor might be worth it for a motor that is started and stopped frequently. Make sure the motor relay is over-sized and easy to replace if possible.
I think a properly sized, totally enclosed, fan cooled three phase motor running directly from the line power is about as reliable a thing as you will find.