Renewable power costs have decreased over time, as there is more capacity installed and technology gets better. This has happened with many industries in history, and the power industry is not the exception. In January, the EIA published the latest cost estimates for plants entering into service in 5 years from now (2018). For instance, you can see that wind power total system levelized costs (which takes into account not only capital and O&M costs but also transmission investment) are lower than conventional coal, nuclear and some natural gas fired power plants.
Now, power generation is just one thing in the energy mix. Electricity consumption accounts generally for about 20 to 30% of total primary energy consumption in a country, meaning that even if it is affordable to build renewables (and we forget about grid constraints) it might not be the unique best strategy to pursue if the goal is to decrease carbon emissions.
But, in all cases, nobody assumes that the quality of life of a person/family/state/country should decrease. In contrast, policies that aim to reduce carbon emissions (i.e. renewables, efficiency in the transport sector, in the industry or in the residential sector, etc) are always considering the increase of the economy (which in many cases is translated into more energy demand).