"The question that comes to my mind is will a tankless water heater last longer than a conventional water heater in terms of rusting out??? It seems to me that replacement water heaters never last as long as the original ones that a builder installs."

The on-demand units are not much larger than my laptop. There isn't a tank that can rust, so it's all pipes and heating elements. I've also looked at having the original tank replaced with a unit made by the same manufacturer. The brand isn't even available for purchase at the retail level, it is only used for new construction or through a contractor.