Common is a simplified version of Thorass, but not in the way that Simple English is a deliberately-simplified version of English, and more like how English is an accidentally-simplified version of Old English.
Thorass was already the trade language of the Realms, and Common simply replaced it. There is no other language that Common is descended from, and it's not an intentionally simplified language. It merely developed organically to be simpler (like real-world Spanish is simpler than its parent, Latin, or English is simpler than Old English). Eventually Thorass itself fell out of common use and “Simple Thorass” replaced Thorass as the trade language and common language of humans. Although its proper name is Thorasta,[1] it's more often called Common because that's what it is — the common speech. (There is an obvious parallel with our own Latin and Vulgar Latin… which was literally named “common speech” in Latin!)
The native speakers of “Complex Common” — Thorass — are still around: humans. Thorass is still used by scholars, and business dealings in Amn and Durpar are still conducted in Thorass[2]:
By the second half of the 14th century DR, few were literate in Thorass, with some exceptions in the southern parts of Faerûn. For example, in Amn, as late as 1370 DR, all official business was still conducted in this archaic language, and even its warriors were required to speak the language. […] Like in Amn, business in Durpar was often conducted in Thorass.
Just like Latin in our own history, Thorass fell out of disuse slowly with the common people (in our history, Latin as spoken by common people became Vulgar Latin, which then itself mutated and split into the various latinate European languages along geographical lines). Common simply replaced Thorass as the, er, common language, and replaced Thorass as the trade language.
As for Thorass being old — yes, it's thousands of years older than Common. Common is hundreds of years old though, and their existence overlaps, so there's nothing odd about that.
As for Common being simple — yes, and that makes it inconvenient as the only language that's spoken by someone. But common Thorasta is typically a human's second language, not first. Their first language is another language like Aglarondan, or a dialect of Thorasta that is more expressively rich (these are the native Common speakers), but only properly understood in limited geographical areas (again, mirroring our own history's Vulgar Latin)[1]:
there were literally thousands of […] local dialects.
In general, the analogy of “Thorass is to English as Common is to Simplified English” is leading you astray. Thorasta is a natural language, not a constructed language. There is no Simplified English equivalent in the Realms — there's no language that was intentionally developed as a simpler version of a current living language in order to make trade easier. Common's place in the world of the Realms is more like Vulgar Latin — a transitional language with many diverse dialects, all more-or-less mutually intelligible, but destined to eventually develop into separate, mutually unintelligible languages. This has even begun to happen: Calant, Kouroo, and Skaevrym are major dialects of Thorasta that are different enough that they have their own distinct identities,[1] even if they are (currently) still mostly mutually intelligible.
The reason that Common is the trade language is not because it was made to be a trade language, but because it is widespread enough (and before it, Thorass was widespread enough) that people learn it on purpose to advance their goals. That's in the same way that today, people learn English in order to participate in global communications, or in the way that ancient Europeans learned Latin in order to communicate, trade, and gain status within the Roman Empire.
