Just thinking out loud, at its core, doesn't “capital” really mean “man-made labour saving devices which are of overall benefit to humanity”?
That enables more people to produce more stuff and thus enjoy their lives more to the overall benefit of everybody.
The notion that capitalism means that each individual must build up “capital” and whoever ends up with the most is the winner is nonsense; capitalism would benefit everybody and should not necessarily lead to inequality – part from the fact that people have different talents and some are luckier than others.
The concepts of saving up during the good times and drawing on the savings in the bad times pre-dates capitalism and is always a good idea on an individual level. As a society, there is no point trying to build up capital faster than new labour saving devices can be developed.
And similarly, anything that is not actually a man-made labour saving device owned by the person who made it, or paid fair value for it, is not capital. Land, patents, monopolies etc are not capital. They are either natural resources or government made; and if the landowner or monopoly holder does not pay the government fair value for the land or the monopoly, that makes them even less like capital as they are of overall detriment to society.
As an illustration:
Consider a farming community who dig the earth with sticks. That's an agrarian society, but they will still save up grain after the harvest, store it and eke it out over the rest of the year.
Some enterprising people work out how to make iron digging implements. So now a couple of people in the village are blacksmiths and the rest are still farmers, but more productive farmers. Each farmer has his own spades and hoes (capital) and the blacksmith has a kiln and tools (capital) for making the spades and hoes.
So the blacksmith has more 'capital' but unless he keeps his technology secret or has his trade protected by guilds and government certification (rent seeking/monopoly behaviour), any excess income he makes would be competed away when his apprentices leave and set up their own business, so everybody in the village ends up with a similar but larger income than before.
If technology does not move on, there is no point everybody trying to build up capital. Why should the farmers buy more spades than they need or the blacksmith produce them? Pointless. There is only a point in building up more capital when somebody invents hose pipes or horse drawn ploughs or something.
If there is a self-appointed landowner who sees that output has increased and his villeins now are making a surplus, he can help himself to that surplus in higher rents. That's where it starts to go wrong, especially if you fall for the lie that land is capital (man made improvements to land, like walls, drainage, good composting etc clearly are “capital” for these purposes, that's quite distinct to what nature made).
In extremis, landowners drive people off the land and they have no choice but to work in factories. The factory as such is a vital part of capitalism, and with equal bargaining power, workers would get a decent wage and the factory owner would get a decent return on his capital. Hooray, everybody wins. But Victorian factory owners could pay ridiculously low wages and live in opulence because the landless peasants, faced with a choice of starving in the countryside or working for a pittance in the towns preferred to take the pittance.
And so on and so forth.