I agree. However, making something look less scary in the beginning still constitutes an improvement from a pedagogical point of view. The more quickly you can learn the basic morphology and lexicon, the sooner you can begin the process of intuiting the higher-level rules and social conventions that govern larger units of discourse.

That is true. One of my pet theories is that at beginner and intermediate levels, simple inflectional morphology fools people into overestimating how good they are, which gives them more courage and confidence to speak actively, and thus helps them improve with time. With more synthetic languages, people are more conscious of how broken their speech is, so they're more afraid and hesitant. But if you somehow manage to eliminate the fear, the advantage of analytic languages disappears.

Due to a large amount of basic structure common to all human language, it's usually not that hard to learn how to sound grammatical. The difficult part of acquiring a new language is learning how to sound idiomatic.

Here I disagree. Even after you learn to sound idiomatic in a foreign language, there will still be some impossibly convoluted issues of grammar (usually syntax) where you'll occasionally make mistakes that make any native speaker cringe at how ungrammatical your utterance is. For example, the definite article and choice of prepositions in English are in this category. Another example are the already mentioned Slavic verbal aspects. (Getting them wrong sounds really awful, but it's almost impossible for non-native speakers, even very proficient ones, to get them right consistently. Gallons of ink have been spent trying to formulate clear and complete rules, without much success.)

I don't know if any work has been done to analyze these issues from an evolutionary perspective, but it seems pretty clear to me that the human brain has an in-built functionality that recognizes even the slightest flaws in pronunciation and grammar characteristic of foreigners and raises a red flag. (This generalizes to all sorts of culture-specific behaviors, of course, including how idiomatic one's speech is.) I strongly suspect that the language of any community, even if it starts as a constructed language optimized for ease of learning by outsiders, will soon naturally develop these shibboleth-generating properties. (These are also important when it comes to different sociolects and registers within a community, of course.)