A system boiler will have a longer life though and will undoubtedly break down less, if at all

Have you got any evidence for that at all? Boiler failure rates for different types from the same manufacturer (not comparing shitty no-name combis fitted by developers against high quality system installs)

As far as i can see the key components inside are pretty much the same -control board, burner, heat exchanger. We had all of those fail on the system boiler in our last house.

Your point on the replacement cost is fair enough but that should be 15+ years away in any case. In London at least 2/3 of the cost of a boiler install seems to be labour

Most unvented cylinders will have a 20+ year guarantee, but realistically they will last 30 maybe up to 40, depending on water softness.

They last for ever, just slowly fill up with limescale. I reckon ours is probably 20% solid limescale (30+ years old, hard water).

From my experience I go to more faults on combi boilers than heat only. I do work in a hard water area though so that would explain some of the reasoning.
Plate heat exchanger, diverter valve, flow switch etc in a combi compared to heat only.