MTG: EDH / Commander

You are correct that Obeka only works if the player whose turn is being cancelled agrees to do so. However, Discontinuity and Glorious End both work during opponent's turns, and do not give your opponent a choice. The idea is that if you use them at the same sort of timing where you would use Stifle, you cancel their triggers and they can't do anything about it (except counterspells).

It's frequently good to run them in Obeka so you have a backup plan in case Obeka is removed, but if Obeka's on the field then you can use her to survive the downside of Glorious end. So basically, it's just a neat trick you can do with cards that might already be found in an obeka deck.