When Silicon Valley billionaires start dabbling in public policy it’s time to go on red alert. Marc Andreessen, investor in Facebook, Pinterest and Twitter, says the universal basic income is “a very interesting idea”. Y Combinator, a start-up fund, is running the numbers on it. Its President, Sam Altman, has already pronounced: the basic income is “inevitable”.

I love inevitability. It’s great for democratic politics. But, while tech entrepreneurs have form on eliding the complexity of the real world (the same Marc Andreesen recently said that anti-colonialism was responsible for an “economic catastrophe” in India), it’s more surprising that some on the left are taking up the idea of a universal basic income too. John McDonnell, the Shadow Chancellor, recently said that the Labour party are looking at it.

Though, wait, this isn’t surprising at all. Supporters of the universal basic income on the left have the same tendency to pretend that the world is simpler than it is, that utopia is right there, agonisingly within reach if only we have the courage to seek it out.

The trouble with the universal basic income is that the utopia it promises is a deceit. Replacing our complex system of welfare benefits with a single equal payment for everyone means one of two things: either the universal basic income is too low to replace the additional benefits people with particular needs receive, for example, those with disabilities or children; or, if it’s high enough not to leave those people out of pocket, then it costs much more than the present system we have.

If it’s the former, then the universal basic income removes money from many of the most vulnerable people in our society If it’s the latter, then it gives extra money to people who don’t need it.

Hang on, advocates of the universal basic income will say: our system can deal with all of that. We’ll taper away the universal basic income for those who don’t need it. We’ll top up for those who have special needs. Okay, brilliant. Welcome to the welfare system we already have.

In other words, when a sensible proponent of the universal basic income starts to get into detailed issues of design, their deceptively simple concept collapses into the usual messiness of government policy. The true utopians though don’t allow themselves to get dragged in this direction. After all, why be distracted by the world as it is.

None of this is to pretend that the welfare system we have is perfect. It’s inherently improvable. People are trying to improve it all the time. Those efforts typically come up against a lack of insight about what life is like for people who rely on benefits; a lack of public money, or perceived lack of public money; and, most prosaically of all, the challenges of implementing complex changes to systems and processes.

From time to time we manage to overcome these issues – sometimes even all three of them at the same time. Proponents of a universal basic income though don’t even seriously think about them, regardless of the fact that their idea is vulnerable to objections on all three grounds. It claims to be universal – so they need to show it works for everyone who relies on benefits at any point in their lives. It’s expensive, in an age when public support for welfare spending is declining. And it requires root and branch change to how we deliver benefits. In case they haven’t noticed, we’re in the middle of such a change in the UK right now: universal credit. Six years on, it’s only starting to work; in some parts of the country; for the very simplest cases, single people with no kids.

But never mind. Proponents of a universal basic income have a utopian idea which they believe is inherently correct. The rest is detail. What a shame that the detail is the messy, shifting, complicated lives of people like us.