You said it yourself:
The fact of the matter is, plastic is non-renewable resource that takes 500-1000 years to break down (see this video for more on the subject). LEGO is a company committed to becoming as environmentally sustainable as possible. They already use plastics to make their core product (bricks). Why would they waste this valuable resource on canisters when their are cheaper and more efficient storage solutions available
Sure, BIONICLE canisters aren't nearly as disposable as other LEGO plastic packaging such as blister packs. Yes, canisters can technically be reused (and most of the early G1 sets had cool functionalities built it). However, I doubt most kids took full advantage of the canister's functionalities and potential for reuse (I'll have to look into this more; it's an interesting question).
Of course it isn't. LEGO is a collective of individual people making decisions. People change. People evolve. To expect everything to stay exactly the same forever (while perfectly understandable) is unrealistic.
LEGO in 2018 isn't the same company producing the same sets you remember so fondly from their childhood. However, if you put down the rose-tinted nostalgia goggles, you'll discover that LEGO is just as full of creativity, innovation and quality as it was 10 years ago. Trust me, I went through a very similar process a few years ago. It's all just part of growing up :).