It really doesn't - the way the 'cost' of the JRS has been described is utterly misleading. It is spending money directly in to the economy and a lot of it will come back to the treasury in any case (people pay income tax on the 80% subject to thresholds for a start). I think it's reasonable to live with it, this is an unprecedented crisis akin to a war type impact on the economy. We need solutions similar to the postwar consensus to stimulate the economy, not the tired austerity orthodoxy of post financial crisis.
It's quantitative easing at a consumer level where it really ought to have been after the financial crisis, it's funny but I don't hear anybody crowing over the QE to banks which has so far cost £645bn to very little economic benefit to the majority (look at the stagnation of wages across the public and private sector) . The headline (misleading as described above) figures for the JRS are chicken feed in comparison.