The biggest thing I took away from this blog post was this.
"NOTE: Basecamp for iPhone requires an account on the new Basecamp (released March 2012). Basecamp Classic is not supported."
I wonder what % of users haven't switched over even after a year. I think this is a good lesson for anyone thinking about effectively launching a new product to replace one they already have.
The company I work for did a similar thing about 15 months ago. We still have about 10% of clients on the old system. We offered free upgrades to get everyone over but some didn't want it. New platform has now diverged sufficiently away from the old one that there is no direct upgrade path any more. This has effectively left 10% of clients stranded.
Looking back I think any replacement product needs to be 100% compatible and after x months clients are forced over. It causes some short term hassle and support costs but in the medium term really simplifies development work if old products are completely retired.