It would make sense to me that, as the social differences decline due to the progresing demographic transition, the societies would tend to loose the the distinction between formal and informal language (not in general but in the sense of special word forms to talk about the person it's being spoken to), just as it'd happend in the evolution of English (where it had been the formal language that had faded out).
I think you are mixing different things.
One is that a stratified society has different degrees of formal education for different people. There consequently is a tendency for the upper layers of such society to speak a different "sociolect" than the lower layers.
The other is that people speak in different registers in different contexts. Anyone, from a beggar to a Supreme Court justice, speaks differently when in court than when among friends in an informal context.
Applying this to your specific question about pronouns, those things may or may not relate.
Taking Portuguese as an example, let's imagine a dialogue between beggar and judge. In court, it should go more or less like this:
Judge: Afinal, o senhor viu ou não viu quando o agressor fugiu?
Beggar: Já disse à senhora: vi, mas 'tava escuro, então não vi direito.
Albeit there is an enormous difference of status between judge and witness, it expected that both call each other a formal pronoun. It would be extremely rude for the witness to call the judge "você", only excusable by ignorance. But it would also be rude for the judge to call the humble witness "você", particularly if it is not a young man or woman.
Now let's imagine a street dialogue between the same characters:
Beggar: 'Cê pode me dar uma moedinha aí, tia?
Judge: Se você cuidar do carro pra mim, na volta te dou um trocado.
Since the context is informal, they may call each other informal pronouns (você and its popular contraction cê). It would be more respectful of both to use formal pronouns, but it is optional.
On the other hand, the use of formal pronouns in informal contexts may be intended as sarcastic, as in parents calling their children o senhor during reprimands (and so, depending on context, the usage can easily be perceived as less respectful than the use of an informal pronoun).
Also, archaic usage often sounds formal to our ears, even though it was intended as informal at the time it was uttered. We tend to hear English "thou" as formal because it is in the Bible and nobody speaks like that anymore; but it was informal, as opposed to formal "you", a few centuries ago.