Tax wealth. I know that's a very hard thing to do due to the mobility and political power of capital, but we need international co-operation and just do it. That's where all the money is and where it's going at an accelerating rate.
E.g. look at Ireland. Ireland has a de-facto wealth tax ("deemed disposal" - taxing the unrealized gains) on ETFs starting at the first euro, which makes investing basically pointless. Meanwhile it also has one of the lower tax levels on property in the EU.
Another thing could be multipliers on property tax. For example, you would pay 0.75× of the property tax on your primary residence, 1.5× the property tax of your secondary residence, and start going exponentially on third, fourth, ... residence. Of course, there would be loopholes (like owning properties through companies), and these loopholes would need to be closed.
You could also tax buy-backs and dividends. (If a company buy their own shares, they have to pay a 50% tax on it)
There are a lot of possible implementations that could add up to each other. It's a fallacy to just point at the bad ones and say "look it doesn't work!"
The main issue is that there has been a transfer of global share income from labour to capital in the last 50 years. However we expect to pay for pensions just by taxing labour. It doesn't matter if there are less young people, because (as shown by GDP growth) they are producing more overall, but misinformed politicians use this argument because they don't understand that these folks didn't profit from the increase in productivity. We need to fairly take a share from this productivity increase and distribute it as pensions. The goal is to pay for what we promised, while relieving burden on young people.
With power, comes the ability to change systems so those with wealth/power do not lose it.
There are exceptions, but that is the rule.
And you only get to do that once.
A realistic wealth tax probably caps out and around 2%, remembering that the net worth of very rich people is generally not liquid and can be difficult to mark to market. Any higher rate risks forcing the rich to have an asset fire sale, which liquidates productive assets and isn't really good for anybody. At six to seven percent you would force them to be entirely liquid and would begin to shrink their wealth, killing off the tax base in a few generations. Even this is absurdly optimistic since a globally standardized wealth tax is a pipe dream and without it you get capital flight as a result of any wealth tax at all, but let's ignore that.
To steelman this as much as possible let's say you can charge 6%. The top 0.5% are worth $30-$40 trillion in the U.S. (which is just an easy example because the statistics are available) which gives you a probably unrealistically high max annual tax revenue of $2.4 trillion. Social Security and Medicare in their current form cost $1.35 trillion a year; Medicare for All would double to triple that or more, depending on what estimate you believe.
Europeans pay for their social safety nets with very broad taxation while America instead charges negative income tax (after credits) to the bottom 60 percent of the population (even after inflation, excise taxes, state tax, payroll tax and tariffs the total tax burden is pretty slim at the bottom end by European standards). There doesn't seem to be a mathematically feasible way to give benefits to the working class that the working class doesn't pay for.
But yes, global co-operation to enable a large enough tax on wealth is not going to happen any time soon. A more likely scenario is that as automation develops and as the share of income that capital gets grows, the working class will end up in a position bad enough that capitalism will collapse and be replaced with something else. Hopefully with something better, but could also be worse.
> Tax wealth
But I don't think even all the nominally socialist parties in Europe are really socialist either.