Some companies are still designing 4bit MCU's in their projects, because they are cheaper, have less power consumption, are more cost efficient.
For simple (and moderate complex) controlling application without high accuracy, wide architectures won't have a performance advantage. their 16/32/64 bit architecture is most of the time not used. So they are expensive, will consumpt a lot of power without doing anything.
For general purpose applications or rapid prototyping bigger and faster might be better to have a fast result about - it's possible or not.
But for cost efficient solutions even more complex problems can be adopted to run on a 8bit machine.
so I think 8bitters will be alive for a long time