After purchasing all the upgrades and characters what is the use of gold

I was frustrated with the same issue. I believe that the lifetime coins challenges do not start counting until after the objective is activated (which could be a bug). You probably activated Gold Miner first, collected nearly 400k coins, and then completed a separate challenge that then activated Midas Touch. From that point all coins collected applied to both Midas Touch and Gold Miner, but because Gold Miner was open first, it has more progress. And, of course, the Midas Touch progress bar is going to fill half as fast as Gold Miner's.

I actually stopped playing after I noticed this. Even though they decreased the +2 Gem Powerup to level 9, it will just take too long to complete the lifetime challenges. If it had been counting my true lifetime progress, they would have been completed already.

It is basically a bug or an unfinished piece of code. From this thread you can see that many other people have it and one of the opinions is:

This is a bug, an error, in the game. A "miser run" is one in which you avoid collecting coins, but in this case no goal number has been set. "Desc" is short for for description. The programer placed this temporary text to remind himself (or someone else) to place description text for the miser run. In the original Temple Run game a Miser Run was running 500 meters without collecting any coins, so either this was simply a repeat performance it it was actually code that was copied over from the former game, but not completed. If you look in the main current objectives list you'll see that it does not show up. I was able to make this objective disappear by starting a run and simply running off the first ledge without jumping.

So don't worry about it, it's just there to take some free space. It might be updated in the future with new objectives(I hope)