Id imagine designers are well aware of the issues of unreadable text for those with vision problems.

Some of these apps are targeting a younger demographic exclusively. If you do stuff that pisses off 25% of your base or makes your app unusable, the complaints will be swift..... then management is on your ass.

I run a few online store..... know exactly when the payment proccessing app or cart is having issues/ unusable becuase my customers will call me.