Do internal SSDs challenge the conventional wisdom of storing digital assets on external "scratch" disks?
Scratch disks should never be external.  I'm not sure where you heard that.  External drives can be used for long term asset storage, but it is best to move assets local to work on them.

The point of scratch disks is to be the highest speed work area you have available to work from.  Scratch disks are used for data that needs to be moved in and out of memory quickly, so internal SSDs are the best bet.

The one thing you don't want to do is you don't want assets and scratch disks on the same magnetic drive because of seek time.  This isn't a concern with SSDs though, so you can generally have them on the same drive.  If you are working with very large files, then there may be a bandwidth advantage to having them on different SSDs, but in order of importance you want memory to be the fastest possible, then scratch disk, then working asset storage, then long term asset storage.

The point is to keep data supplied to the CPU and GPU as much as possible.