It hasn't held them back. I wonder if it's a case of their services been so unique, cheap, competitive, etc. or if we overrate design and it's not really that important. I tend to think it's the former, but it's an interesting example.
My issue isn't so much with design but rather the fact that you have to register a new account with them every time you want to use a new service. This is almost certainly legacy complexity from the days that these services were not exposed through API's, but it is still a pain in the ass.