I must admit to being confused by this comment. What do you mean exactly by ‘all words having a declension’, and why would it complicate learning a language moreso than anything else? My confusion arises from the fact that the concept of a ‘declension’ is typically only applied to noun-y things, rather than to all words (the verbal equivalent is ‘conjugation’), and by this definition a ‘declension’ doesn’t even have to be very complicated — the English plural is an example.

(Unless you’re talking about declension classes, as in Latin or Russian… but from what I can tell, those are actually surprisingly rare outside IE.)