While these backs certainly will not last forever, I think that unlike mechanical shutters, the exposure count does not necessarily indicate anything other than how often the back was used... I would think that factors such as abuse, time spent in extreme temperatures, and storage conditions would have a much bigger impact on the life expectancy than the amount the of exposures.
(Spoken from someone who is happily enjoying an 11 year old Aptus 22 with a few hundred k exposures and still going strong. )