There are a few situations where the "T" pronoun in a T-V distinction (e.g. thou/you, ty/vy, tú/usted...) might become socially proscribed and avoided:
- if its usage implies that the hearer is below the speaker in the social hierarchy, in a society where drawing attention upon this would be considered belittling.
- if higher class speakers already use the "V" pronoun among themselves, even when there's no hierarchy between them. As such, the usage of the "T" pronoun becomes associated with the lower classes, and stigmatised.
- if the "T" pronoun implies more familiarity than people are generally comfortable with, making its usage feel invasive towards someone's personal space.
English likely lost "thou" due to a mix of those three factors.
Regarding Portuguese, it's worth to note that even in Brazil plenty speakers use "tu", depending on their actual dialects; for example the pronoun is practically non-existent in Paulistano dialect, but commonplace in Gaúcho. In the dialects that lost it, the process was likely similar as in English (i.e. "tu" was seen as too familiar, belittling, and something that stigmatised groups would use). The language eventually redeveloped the distinction with "o senhor" (roughly "sir") as the new respectful pronoun, and in a few dialects (as in e.g. Santa Catarina) you see a three-way distinction between:
- tu - familiar, non-hierarchical
- você - distant, non-hierarchical
- o senhor - hierarchical
Because of that, even with how common "você" has become, it's possible that "tu" will still survive for longer, as there is no pressure to remove it for sounding belittling or "low class" any more.