[thaumasiotes]

> but modern English is almost completely uninflected

That really depends what you're counting; if you ask which words in a sentence must be inflected, English still looks highly inflected. If you compare it to other inflected languages, the number of distinct forms for any given word is low, but this doesn't do much to help learners coming from languages that don't observe the same distinctions that require inflection in English. They don't do a lot better choosing between the fivish forms of an English verb than between the dozens of forms of a Latin verb.

> dual has been lost in most IE languages

This is a good tangential example; the dual persists, in reduced form, in modern English! We don't inflect verbs for it. But we do inflect determiners; we scrupulously distinguish both from all in a manner that most people in the world find confusing.