I'll agree with your basic point that reduction of poverty is, in some sense, the correct measure to look at. However, the problem with applying your stated attitude to shaping policy is that society has a limited ability to innovate or grow productivity on a finite timescale. In other words, it makes some sense to think of economic productivity as relatively constant (it's unlikely, e.g., that someone will figure out how to conjure up calories and end world hunger next year).

On the other side, people are suffering right now. If we agree that reducing this suffering is the correct measure, then we must ask ourselves what steps can be taken right now to lessen it.

If we were in an intellectual vacuum, then we could argue all day about the pros and cons of wealth redistribution (and whether or not it is theft to apply such a tool). But what many (including myself) feel to be true, is that now more than ever, a lot of the productive capacity of money is being needlessly accrued by relatively few people who, on average, don't possess the the knowledge or will to apply that capacity to society's problems. Under those conditions, the potential for growth and productivity in the future is needlessly limited.

It is one thing to say that the gatekeepers of society's wealth function as a storehouse of capital for future contingencies, but this largely ignores that our global monetary system is primarily a fiat one, and as such, the integrity of the society itself and peoples' trust in the system is of paramount concern.

TL;DR The productivity of today's capital is being hindered by accumulation because the long-term benefits of more equitable distribution are philosophically real, but hard to understand.