The difference is that Scala devs didn't push for immediate adoption while they were still working things out.

Apple's Swift message is more or less "Stop writing Objective-C if you can and start using Swift – oh and by the way – you will have to rewrite your code with every major Swift release which we will be releasing at a rapid schedule".

It will be very hard for Swift to get rid of all the cruft they accumulate.

In Scala every major release addresses one or two pain points and migration is very smooth – no "rewrite all your stuff".