This is very disturbing to me, and I suspect to a lot of others. Significant changes were made to the lesson format- but why do I need to go back and complete more lessons in categories that were already completed before these changes were added? A little common sense and fairness would be useful here.
You might not agree with it, but a lot of people are here to work on their language, not on their skill tree. Your skill tree may have become less complete, but the changes were introduced to allow you to get a more complete knowledge of the language itself.
Why not learn something new? Basic skills are simple and it won't take you too much time to do the additional lessons. It provides you an easy way to earn some skill points while brushing up on basic conceptions. Those skills that are harder are definitely worth learning, aren't they?
I don't think you only want to learn some fixed set of words. Language is not "a set of skills offered on Duolingo", but much more.