I could imagine the very high costs to be a big reason for people not to tank but to play dps
Cost was higher before and we had more tanks back then. Outside of most flashpoints without tanks there is no endgame content.
BioWare releases good, challenging, regular progrssion content and repair cost will have nothing to do with the number of tanks. Tanks will be there because without all three; tanks, healers and dps, there is no way to do progression content.