Except for the fact that it doesn't take 5 years. People who are running DR:Mentor once a day for the bonus aren't the people this was designed for. I would argue it was designed for people like me who run DR:Mentor 1-5 times or more a day. I don't run DR:Mentor for titles or for achievements, I run it to assist people with clears. While a lot of the EX primals do end in failure for one reason or another, they don't always end in failure. I also stick through every single one until the timer runs out, a vote abandoned is passed, or the trial is cleared. I've had quite a few of the clears come in when the timer is down to the final minutes and sometimes even seconds.
Based on my current rate, I could easily obtain this mount in another 18 months, not 5 years like people are QQing about. That is a conservative estimate as I've taken the average/day of successful DR:Mentor runs based on my current total and dropped that number down. Even without the mount as a reward, I would still be running DR: Mentor frequently as my primary objective is to help people.
Essentially I'm trying to point out that not every reward placed into the game is meant to be obtained by every player doing that content. This is an example of a reward designed to be awarded to people who go above and beyond the standard without them even asking for it. Sure, a lot of people are going to try and grind this, most will probably burn out pretty fast, some will make it to the end. However, the rest of us will continue doing what we do and probably won't even know we are close to obtaining it until the notification pops up on screen that we hit that achievement.
tldr: This will only take 5 years to obtain for people who do the bare minimum, it will be obtained much faster by those who are actually running DR:Mentor to help people and not for the rewards associated with it.