Actually, there's really no possibility for the steering shaft to pull out of the coupler when its installed, because there is virtually no front-to-back movement. The large split pin driven through the lower end keeps the coupler fastened to the steering gear box's splined output shaft. The upper end of the steering shaft is held in the column housing with another large solid pin. And the whole column assembly is bolted to the firewall.
I have always understood that the coupler was designed to be a flexible connection between the shaft and the gear box to minimize vibration to the steering wheel, and in case of a front end collision, it canl absorb some of the impact.
