How can such a simple idea be screwed up and bastardised so badly?
Because the underlying systems - a hotchpotch of various benefit and taxation schemes and systems was - charitably - not fit for purpose.
The whole sorry saga is reminiscent of the Blackadder trope of fixing wheels to a tomato. "Time consuming and utterly pointless".
The right approach would be to (a) rationalise tax and benefits and (b) then get a system to automate it.
The only way (and you read it here first) UC will be killed (no, not a stake through the heart) is when the government (any one) has a "review" and discovers that "due to other changes, efficiencies, better ways of working, etc etc" there is no longer a need for universal credit as "it can be delivered in other ways" before being quietly led down a Whitehall alley and given a fare-thee-well bullet through back of the head. (NOTE. This approach also guarantees gongs for all involved).