i only finished the main quest, the brotherhood, wizard, werewolf and thieve's guild, and the daedric quests.
While I wont say your wrong, as yes I could see 72 hours being enough to "win" the main quest ( and miss about 80% of the games content) as most games today are 10-30 hours to complete 100%. This game is about exploring, finding little tidbits/stories that do not pertain to the main quest line. And again i get it, some people need a path to follow and specific finish to end a game, nothing wrong with that. In my opinion your missing out, but in yours its maybe just a waste of time. I have played the game through to "almost" completion 3 times, mage, unarmed, and thief builds. I never finish the game as I too would feel its "over" and wouldn't do the side quests.
As for me I preloaded and am excited to try a sword and Board build, but really want to build a mage again.. maybe 2 play through s with the new updated graphics?