A great deal of UC expenditure depends on the personal circumstances of each claimant, including income from others in the household, housing costs, number of children, childcare costs, etc etc. Unless we are going to ask employers to vary wages in a similar manner, or pay everyone a lot more whether they need it or not, I'm not sure how a living wage is going to remove the need for that expenditure.
You cannot totally remove the need for a benefit which tops up low incomes.
For those obliged to rent privately there's a massive difference between the amounts paid for housing depending on where in the country you live. There will also be people who have family/care commitments or health issues that limit their working hours.
The efficacy of increased pay in lifting real life living standards would be greater if the taper rate for Universal Credit were better; claimants only see 37p of every £1.
It is also worth pointing out that the uplift raised the Standard Allowance in Universal Credit to that it matched the rate for Statutory Sick Pay; a benefit which is itself pretty miserly. When SSP was introduced in the eighties, transferring responsibility for sick pay to employers, its cash value was broadly similar to that of Unemployment Benefit. SSP was usually linked to RPI/CPI whereas benefits paid directly by the DWP have been subject to freezes and below cost of living increases.
In other words the uplift did nothing more than restore its purchasing power to an earlier level.
Yes it absolutely should be made permanent. That it is not shows how little actual commitment the government has levelling up real world experience.