Become? The problem here is that we already are far beyond what is healthy. Air quality tends to be worse in China than in Europe, but a big difference is also that the Chinese tend to be less hesitant about wearing masks when it's important for the health.

The air is actually getting better (both in Europe and in China), but it will likely take decades before it's within WHO limits. Hence for the foreseeable future it makes sense to consider masks and air cleaners as an option. I'm not saying it's a good idea to wear a mask 24/7, but there's quite a few places and times in Europe where I'd put one on. There's real time data (e.g. here for Germany) and certain weather conditions exacerbate the problem, so it should be possible to avoid most issues with relatively little mask wearing.

The only issue with normal (FFP1-FFP3) masks is that they only work against particulates. For other issues (SOx, NOx, Ozone) they don't do much. Though afaik air cleaners with activated charcoal are helpful in these cases.