I've seen heaps of photos where people have chopped their Beauty Blender in half and found all sorts of gross things inside (even bugs, gross!) My Beauty Blender is six years old and very well used. It's all discoloured now and I barely use it any more, so I decided to cut it in half and see what was inside. I was expecting it to be really horrible so was pleasantly surprised to see that it was clean.
Here's the thing. I never hand washed my Beauty Blender. I'd use it all week and at the end of the week I'd throw it in the washing machine. I put it in a lingerie bag and washed it on the warm or hot cycle with my towels. That's it.
I think that's the secret to it's longevity, not hand washing means it wasn't roughed up so it lasted for ages. I could have kept on using it, there was nothing wrong with it, I just assumed it'd be gross after so long.