According to the article the conversion kit costs $100k and it has no mention to lifespan.
But say your lifespan number is correct, lets look at this.
8 hrs a day, 5 days a week, 52 weeks a year for a fulltime 100% driving duty vehicle = 2080 hours/year.
So already this conversion would last 5 years at a bare minimum. Please note the article suggests a 3 year payback time on this, meaning a possibility of an extra $66k of savings for the medium duty kit.
Of course that would require the turbine to be running full time and the vehicle to be in motion the full time. Which is not the workload they are targeting.
For a garbage truck spending some of its work time parked to pickup dumpsters and most of its driving being start/stop (regenerative braking), lets say the turbine only runs 66% of the time. That would give a converted garbage hauler a 7+ year engine lifespan, which is frankly pretty darn good.
A delivery truck with a maybe 50% drive time would last over 10 years.