From a sociolinguistic view, it seems likely that if Esperanto were the universal language, minority-language speakers would abandon their native languages for it in many circumstances ("domain loss") just as they do for English, French, Spanish, etc. in various parts of the world.
I think you're doing him a bit of a disservice here. His idea wasn't to kill other languages at all. He felt that if you only had to learn Esperanto to communicate with the rest of the world, you would, in fact, be able to spend more time enriching your own language. As he wrote in "Unua Libro":
"How different would the case be, had we but two languages to learn; we should know them infinitely better, and the languages themselves would grow richer, and reach a higher degrees of perfection than is found in any of those now existing"
I'm not sure why irregularity would emerge from lossy transmission. I would think regularity would be a greater benefit here. I assume it's just a historical accident that arises when verbs stand on their own and you don't have to do anything with them. When you're just starting to have language, it must take some real insight to realize that there is a special relationship between "go" and "went", and it's the same as the relationship between "be" and "was". But if you regularly (ha) need to agglutinate a pile of endings on your verbs, then using unique words for each tense would quickly become a pain in the butt.
I find agglutinative languages are simpler than other types of languages in many ways, and I haven't studied linguistics enough to know if that's inherent to all of them or just coincidence from the small number of such languages I've studied. Turkish plurals are perfectly regular (there are 2 possible endings, depending on the word's final vowel), and Japanese doesn't really have plurals at all. Neither language has gendered words, or definite articles. Turkish has no diphthongs, and Japanese doesn't really (depending on your POV), either.
Esperanto actually looks somewhat complex to me! It reminds me of my own (failed) attempt to learn French. It seems to have a lot of baggage from the Romance languages, presumably to make it seem more familiar to those speakers. I'm a little surprised it wasn't simplified further.
My layperson's view on it is that "everything is regularised" means "no convenient shortcuts" for the benefit of "anyone around the world can use it". That's a huge problem for day to day use when you and your friends have to willingly choose to say an inconvenient sentence over and over and over, just so that a person from a thousand miles away could understand you if they were ever here. It's a bit of a tragedy of the commons - you do what's more convenient locally in the short term, and the long term wider scope suffers for it.
Esperanto's original ideas were to have many compound words - make them up on the fly, to keep the vocabulary load smaller. It took off early in French speaking regions and gained a lot of French-root loan words, and then got compound word alternatives, so there ended up being multiple ways to say the same thing. e.g. eyeglasses are okulvitroj (eye + glasses). They're also okulvitro (singular) because imperfection. And they are nazumo, from pince-nez, which is nose+"related in a non-specific way" suffix. Then you get really weird words built up from suffix games like "estonteco" which is a "play with words" way of saying "the future"; there's no way I'm going to create or deconstruct that on the fly, possibly no way you'd work it out without learning it.
In fact, now that I think about it, Zamenhof may have calqued the Esperanto word specifically on Russian будущее "that which is to be", i.e. " the future".
The only downside I can think of is that we're taught in school that slightly different spelling etc. is strictly incorrect, so there's a stigma you'd somehow have to overcome for the native speakers.
Bear in mind that ⅔ of all the English speakers (over a billion people) are second language users of the language.
Of course what you are left with is a language that doesn't really reach its potential for beautiful prose until you settle for a specific 'proper' variant of English — with all its bells and whistles, comparatively fixed orthography, and quaint expressions.
I am a long since ex- Esperantist who is not keen to defend the language and movement, but your post is way off the mark. Esperanto was a decently big fad in the former Eastern Bloc, because learning it allowed you to correspond with exotic foreigners and maybe even get visa invitations to go visit other countries (for Esperanto events) you otherwise might never dream of. Sure, some of those countries are now much more prosperous and developed, but they weren’t then. My memories of European Esperanto congresses decades ago are a lot of Ukrainians, Russians, Poles, Hungarians, and Romanians clearly on the lookout for a Western European spouse, or stuffing their suitcases with local goods they could sell back home.
Similarly, West Africa has always had its own local Esperanto scene, for roughly the same reasons of being able to get visas etc.
There was a lively Esperanto community in Yugoslavia, which was a non-aligned country that had more travel opportunities, but still wasn’t a "rich Western country".
You hail English as "able to give the underprivileged greater opportunities". But the reason for that is that English is more widely spread, and dominant in the most wealthy areas. Which is... well... imperialism.
It sounds like you're simply dismissing Esperanto as a niche hobby for a handful of white nerds. Which, in terms of practical benefit, might well be accurate. But it is BIZARRE to invert the definition of imperialism to over-state this point.
Anecdote: We had a Japanese Esperantist visiting a local meeting in the US and of course asked if Esperanto is really ten times easier than English for Japanese speakers.
His reply (in Esperanto): "No. It's a hundred times easier."