Plus, apostrophes are cool! How're you goin' to signify a contraction or indicate possession within a language that's had these grammatical conventions for centuries?
Oh, I agree, but there is a disturbing number of people who think that making a noun possessive in English is the same as making it plural. I get very, very sad every time I see someone write something like, "the trucks tire went flat." See, it really ought to have an apostrophe there, but many people don't understand that and apparently get by thinking that this is how it works. People do the opposite too, like writing, "I bought a pair of shoe's." That one is even more bizarre but it still happens and people technically understand... And it seems that about 75% of people don't know the difference between its and it's, so those might as well be the same word, you know?
It's almost becoming the rule
because of the fact that so many people misuse the apostrophe and technically understand what was meant. I forget the term for things that become accepted practice despite being technically incorrect, and I'm pretty sure there is a term for that.
There are some other language features that seem a bit unnecessary too. Capital letters, for example. They signify the start of a sentence and I guess may make it easier to read a paragraph, but their use for proper nouns seems a bit pointless.
Anyway, rant over! Yeah, you're probably very correct about why Esperanto has remained fairly pure over time, and I'd be interested to see how it mutated if it was very commonly spoken. The rise of the internet makes it much more available and easier to get access to learning material, but the motivation to do so is still pretty much nonexistent so I doubt that's going to happen. English has largely taken the place of Esperanto's goal of being an international language, and it's going to be hard to displace it.