The thing in FranP is that there hasn't been been a massive uptake in pension credit. Only 6% of those 760,000 households said to be eligible have made a successful claim since last July- just 45,800 from 760,000 households. That leaves over 700,000 households not claiming for whatever reason and as much as £1.5 billion Pension Credit going unclaimed.
If the limit to pay WFP now is set too low, say at £20,000 as someone else suggested, it means that millions of pensioners with relatively low incomes will have to self assess for tax so that the payment can be clawed back. Many won't find that an easy process.
It could be argued that a large proportion of people in the £35,000+ income bracket already do self-assess so they are in the system and know what to do each year.
The administrative load and costs are a major factor in this.
Means testing is hugely expensive. It's hard to find numbers but 15 years ago it was costing £450 a case to means test for Pension Credit.
Far easier to pay WFP to every household and claw back through tax. Many may chose to opt out.
I think this is a decent compromise.