Should Duolingo estimate how long it will take you to master the entire skill tree based on how much progress you have made in the last week or so?
No. It's impossible to tell how long it will take someone to learn the complexities of a language based on how good they are at the basics. Besides, someone who takes their time and learns properly will probably do better in the long run than someone who rushes through. An "estimated completion time" would only set up for disappointment or put people off.
Edbaker is 100% right in this case. I also want to add that 'mastering' the skill tree does not necessarily mean that you learned the language and can use it in everyday situations - quite the opposite,really. Adding a 'predicted completion' bar would only add to this illusion.