[otoburb]

The more people there are in the world, the larger the supply of labour. The more technology, (usually) the more productive people can be. This means that people from developing (i.e. poorer) countries are able to do more at lower prices, forcing everybody else to work harder overall to keep up.

It's initially counter-intuitive, but ultimately this is supply and demand. You can coast, but while you do, other people with higher incentives (e.g. more desperation to get out of their poorer living conditions) are working harder.

[Rexxar]

We have now more technology and more people than in the 19th century and people work less and have higher salary. Why would it be different in the future ?