The poor ending was entirely due to the hubris of Dumb and Dumber who were greedily looking ahead to their next projects. HBO was willing to give them as many seasons as they wanted after season 6 and they said, "Nah, we can do it in two seasons. And instead of ten episodes each they’ll have 7 and 6 episodes because “Fuck You”. They could have handed it off to someone passionate if they were bored, but again, hubris.
8+8 and 8X2 are literally the exact same thing, why did they feel the need to make that an extra step?