In my experience, it seems to be that people learning as a second language one that is significantly more inflected that their mother tongue(s) experience serious difficulties and tend to avoid flection in informal speech. That happens even nowadays, when flection is often taught in a formal way.
Judging from this point of view, does it really sound feasible that Bulgars, Avars and other Turkic-speaking peoples might have given birth, together with Indo-European peoples, birth to Proto-Slavic as a lingua franca or pidgin to communicate between different themselves? I mean - how Bulgars and other Turkic speaking people give shape to a fully inflected language with almost no loss of inflection complexity with regard to, say, Baltic languages?
I can't see how non-native speakers keep up to that level of inflection.
AFAIK Middle English lost much of the inflection as a result of Old Norse and Old Saxon speakers trying to find a common language and having to remove away the obstacle of different inflection systems.
That (Proto-Slavic as a made lingua franca) is a thesis proposed by Florin Curta. I'm aware that it's subject to much criticism and I wonder if the point above could be an argument.
(I'm just an amateur.)