Suppose that all wages, salaries, welfare benefit and sources of income were indexed to inflation?

yes. inflation/deflation impact on consumers behaviors. they are an averaged % based on a basket of goods. however, people's purchase mix can differ from person to person even if on average they drive the inflation basket. therefore the pressure up on some products may not be accompanied by the same pressure down on some other. if we can assume that there will be a perfect correlation between the two, yes, the problem would be less important in FMCG area. However, there are some other prices which are not part of inflation, as houses prices for example. Depending on its specific market dynamics this would drive specific consumers behaviors, that could impact on FMCG prices. So, in my view, in summary such a correlation would partly (at most) diminish the impact of inflation.