I think that Williams is quite deliberate in concluding the play in a manner where there are no "winners."  Blanche's institutionalization is not done for her own welfare, but to consolidate Stanley's own control over his wife, something that she herself regrets in the end.  Stanley is shown to be driven by power and whose actions are brutish, revealed in pure atrocity.  Williams is able to show that the resulting collision of values in the modern setting does not yield a winner, but rather different shades of loss. Perhaps, Blanche was not made for her particular setting, but her confinement and ostracizing are shown to not be proportionate responses.  In the end, the tragedy is for everyone concerned.  Stella understands her failure and Stanley's failure as a husband.  Blanche is forever confined with her melancholy, unable to articulate her condition and relegated to silence.  Stanley has gained control, but the loss of faith from his wife is demonstrative of reflecting power over nothing.  In the end, Williams' construct of modernity is one where pursuits of happiness are evident, with happiness being far from it.