The way I see it, capitalism is an environment. A ‘place’.
A ‘way’ for people to do ‘economy’.
What people do in that place depends on the place itself but also on how they choose to do things. This being the reason for which the American capitalism is different from the European one. And both completely different from the Chinese version.
In this sense, capitalism doesn’t actually work. Not by itself!
If those dwelling in this ‘place’ act freely – as in ‘free market’ – then the whole ‘thing’ remains ‘sustainable’. Not ‘good for everybody’, not always ‘nice’ but nevertheless ‘fair’. As in ‘you have a fair chance of reaching the other end’. Not to get necessarily rich but to make the ends meet!

The alternative to capitalism… if you take your ideological blinders off, you notice that there’s none!
Socialist/communist countries are/were also capitalist. The difference being that their economies are/were centrally planned. Their markets are/were anything but free!
This being the reason for which communism had crumbled under its own weight.
And for which in all places where the market is not free enough the ‘thing’ is not sustainable!