The roots are shriveling because the media is bad. When it is bad it holds too much moisture and the roots die. The roots need air.
When a Phalaenopsis is in trouble like yours, you repot it! They actually enjoy being repotted and recover quickly!
It keeps growing because basically it is happy. But to keep it happy, it needs to be repotted every 2 years, faithfully.
Use a new pot that the roots comfortable fit into. NOT TOO large. Then use fresh media. Bark mixes work well. Fir bark with charcoal and sponge rock is great to use.
Welcome to the site!! Check out our threads in the Orchid Forums. You'll find loads of good information. Read the one on Supermarkets Orchids-Phalaenopsis!
Trim off dead roots, dust with cinnamon.
