Ok, you didn't initially specify that, you just said to end housing benefit altogether (which I took to also mean UC housing element, since the only people getting housing benefit are OAPs now) and to only have the basic UC and state pension.
So according to you, someone who has been made redundant or who is too unwell to work or who is disabled or has a disabled child, but who is 18-55 and doesn't have a "severe disability" has to die - unless they have friends or family to take them in and let them sleep on their floors if they have no spare bedrooms. Just because you think help with rent and any other "extras" should be abolished. Nice. Some might survive as street homeless for a while, but that is known to shorten life and make it almost impossible for someone to get a job too.
It would take money to implement your suggestion, there's currently no automatic screening for the criteria "18-55 and not severely disabled".
More money in the form of NHS costs and disability benefits for those who became "severely disabled", due to having basic necessary expenses double or triple their income and becoming homeless as a result.
Yet more money housing all those children, belonging to those adults, in orphanages. Along with the NHS costs from the harm that would cause. And the ongoing costs mentioned immediately above, when those children don't grow up to reach their full potential and become the type of people without the best education or health, who struggles to find employment.
Or perhaps you think we shouldn't build the orphanages and the children should be left to die homeless too. Especially the disabled ones who's parents can't work because they're the full time carer, but they come under your heading of "18-55 without severe disability" themselves, so no rental help for them, just £400/month UC, not even the £80/wk additional carer's allowance - because that's an "extra" beyond the basic allowance so according to your plan it wouldn't exist - to pay for absolutely everything, which is obviously not doable.
It makes no sense as a society to incur the costs of someone who is long term homeless until death, but who could have gone back into the workforce after a time as a jobseeker, just because you didn't want to allow help with rent during that time.
I suspect your "family planning decisions" parents would be forced to make would include illegal euthanasia of their disabled or young kids, since there's no way they can support them and look after them, especially if they're a single parent.
Lots more suicide as anyone currently renting and diagnosed with anything that means they can't get back to work after a few weeks (assuming they're diagnosed in a timely manner in the first place) would realise they're faced with imminent homelessness. Those as yet undiagnosed, or just unfortunate enough to have lost their job due to redundancy, also either ending their own lives, or alternatively becoming lawless thieves to obtain what they need, due to society labelling them worthless and blaming them for their own circumstances. Many wouldn't feel particularly as if they owed anyone else basic decency, when others aren't treating them with basic decency.
As I said, your idea would be a disaster that was ineffective for society, as well as all the individual people involved.
I shan't engage with you further since it's clear I'm also right about you thinking majority of benefits claimants are scroungers.
You're also attempting to derail the thread since it's about pensions and you're talking about benefits reforms for 18-55 year olds.