Dempsey Roll is the best technique suited for a boxer like Ippo. Coach can’t just train Ippo to be a counter puncher or something. So he was delaying the fight until Ippo gets all his “stats” (attack, defence etc) higher. Imagine seeing Mike Tyson trying to jab and counter, just wouldn’t be effective. Same applies to Ippo.
Coach is harsh because he’s worried if Ippo fails then he might just give up or be so mentally destroyed if he loses that he won’t be able to recover.