Hardened valve seats - do they really matter?

potentially run at max torque for hours and hours, great potential for oil longevity provided you have good rings and a filter. i.e no short runs spend most if its running life at optimum temperature, any in-block condensation driven off pretty smartly every use, Being on or close to the water you'd have to expect some humidity and dampness when the sun went down

but with little or no on off the throttle the variation of what pases through the inlet and exhaust would not match the variation seen in street driving...
the longer the valve is on the seat the more heat it can shed to the head and coolant
4000 rpm for 4 hours gives no variation is seat time, exhaust valves probably glowing but Not being cooled enough by intake charge.
no accelerator pump action gives no variation in burn no variation in unburnt fuel going out the exhaust.

i'm guessing in an ideal world you would swap to a cam suited to marine use
if a marine use cam is anything like an LPG cam then it would have specific lobe shape to deal with the valve and seat impact (in both senses of the word) caused by the change in use

quick open and gentle landing is I think what an LPG cam has (LPG burns hot has no additives and can cause seat recession)

i know nothing about marine engines but i would suspect any issues he had could be tamed by a cam and springs for the job, that focused on max torque at mid range rpm.
make best use of the fuel you have and not be wringing the neck of a motor that has to get you home without the use of an SOS flare and your radio.

or as mentioned earlier stainless valves into an iron-head seat, different materials that don't like to stick together when they slam together ...less micro welding so less seat recession

you can friction weld two pieces of the same bar together easier than two pieces of different material
spin speed or pressure or both need to change to be effective in both cases and id suggest greater friction is probably necessary in the second case ferrous metals and those used in alloying ferrous metals

Iron based standard valve head and nodular iron seat, work or induction hardened, need chemicals in play, to stop them doing it. tetra ethyl lead or some modern alternative.

standard valves seat recession problem
ferrea stainless (5000 6000 series) less chance of a seat recession problem
spend the money you saved on the seat inserts on the valves and use em in the next 3 engines :)

Dave