The World Economic Forum predicts a USD 400 trillion pensions shortfall by 2050

I mean, what would be the point of savings if the real economy is less productive?

I would highly recommend reading The Road To Serfdom (https://en.wikipedia.org/wiki/The_Road_to_Serfdom). It details precisely why a right wing government needs to invest in welfare and social care for this exact reason; when a government fails the poor (or the poor's parents I imagine) they have a tendency to rise up and kill all the rich people.

W climbs trees every day to fetch coconuts and then swims them across the channel to a neighboring island where she barters them away for drinking water and then swims back with jugs full of water.

P used to help W with the tree climbing, but is now too old to do the work. However, he long ago made a deal with W, and she's a man of her word, so she keeps him supplied with enough fresh water to get by every day. Still, when the yield of coconuts becomes bad, or when there's a glut of coconuts on the neighboring island, she is liable to feel that there is an excess of pensioners on the island.

U is in desperate need of water and would do almost anything, but he doesn't know how to swim or climb trees.

Now, tell me: How does the existence of U help get the fresh water that P needs? How does the existence of P help U find a way to earn his water?

The reason is that republicans are referring to accrual accounting, while others are looking at the balance sheet. It's very uncommon to do accrual accounting in government, though I've heard rumors that military pensions are funded in this way. My understanding is that, outside the government, it is obligatory (de facto or legally, I don't know). The absence of accrual accounting is known as a Ponzi scheme in the private sector.

When you let governments reap the benefits now and pay later, they use unfunded pensions as compensation and build low-value roads to cheaply built houses built in corn fields. And for government pensions, it's impossible to sustain the required infinite growth in a finite world. In the short term, the tendency of developed nations toward low birth rates breaks the systems even before we reach space concerns.