Twitter recently started rolling out their new "improved" redesign a few days ago and it has been met with an extremely negative response by the community. Not only are certain features completely broken, but the whole design itself seems to violate the basic rules on website design and usability. It is clear the design is aimed for tablets and smartphones, making use of overly large fonts and occupying only a third of the screen. However Twitter has clearly failed to realise that the majority of users of Twitter on desktop do not use touch screens. Using this design with a keyboard and mouse therefore feels clunky and unnatural. 

Giving users the ability to revert back to the old design (a feature that was previously present but was later removed) will allow the Twitter user base to decide for themselves which design they want to use. This idea has been proven to work before, perhaps the most famous example being Reddit, which has allowed users to switch between its old and new designs for the past year. There is absolutely no harm in allowing users to choose, and requires little effort on Twitter's end to implement. That is why Twitter must allow users to revert back to the old design.