It's the statist idea. Socialism in practice usually involves regulating the market heavily, or into oblivion altogether, and giving the State a huge redistribution power. See my comment nearby on why such a setup fails to work.
A socialism where the only way to work is to own a part of an enterprise (so no "exploitation"is possible) would likely work much better, and not even require a huge state. It would be rather inflexible though, or mutate back into capitalism as some workers would accumulate larger shares of enterprises.
Having some kind of default steward for market developments that get so competitive and fundamental that they reach full market saturation is helpful. Under a market system, at that scale, the need for growth starts to motivate companies to cut corners or squeeze their customer base to keep the numbers going up. You either end up pricing everyone out (fixed supply case) or the profit margins get so slim that only a massive conglomerate can break even (insatiable demand case). This is why making fundamental needs and infrastructure into market commodities doesn't work either.
The problem with social democracy is that it still gives capitalists a seat at the table and doesn't address the fundamental issues of empowering market radicalism. Some balance would be nice, but I don't really see that happening.
"market radicalism" as in, the notion that ~all aspects of life should be subject to market forces. I think markets are good for a lot of things, but competing for basic necessities like food and shelter puts people in a really unhealthy headspace. The green revolution made a lot of our concepts about food scarcity obsolete.
A good example of market radicalism at play in the US is the healthcare system. "Everyone knows" that the market is better at allocating resources, but we actually have terrible outcomes and there is no political will to change an obviously broken system. Despite having real-world examples of single-payer (and hybrid) healthcare systems out there that are more cost effective and have better outcomes.
On what market the cost of services to be rendered would be held secret, or at least obscure? Try shopping for a particular kind of surgery across hospitals. How many plans does your employer offer? Would you rather buy a plan independently? Why?
The whole "insurance" system is not insurance mostly, but a payment scheme, of a quite bizarre, complicated, and opaque kind. The regulatory forces that gave birth to it are at least as strong as any market forces remaining in the space, and likely stronger.
OTOH direct markets are a poor way to distribute goods that have a very inelastic and time-sensitive demand, like firefighting, urgent healthcare, or law enforcement. Real insurance is a better way, and a permanent force sustained by tax money usually demonstrates the best outcomes. The less urgent healthcare is, the better a market works, all the way to fitness clubs which are 100% market-based.
A socialism where the only way to work is to own a part of an enterprise (so no "exploitation"is possible) would likely work much better, and not even require a huge state. It would be rather inflexible though, or mutate back into capitalism as some workers would accumulate larger shares of enterprises.