i never noticed that when i watched the older episodes, and it certainly didn't stop any of my little brothers or sisters (i have lot of them) from understanding it. as for the life and learning lessons being less complex in the older episodes, i think it is actually the other way around. the newer episodes become less complex entirely, going from, say, wishing you were more mature, or being poor when you have a rich friend, to cupcakes being stolen. (although i do like some of the newer episodes, and a few of them do have good "life lessons," a lot of them are just random time wasters.
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but i understand what you're saying, that's a problem with a lot of kids shows made in the eighties that are still making episodes. usually cartoons back then were not well made at all, (Arthur is an exception.) so they had to try and live down a lot of criticism by making "better" episodes, and hiding the old ones. Arthur seems to be doing the same thing even though the older episodes were some of the best. the new ones aren't bad, and i think they will get better with every episode, (hopefully.) but they certainly aren't better than the older ones.
kudos if you read the whole thing.