IMO, there's no hope of automatic PNG to (useful) SVG, seeing that that would involve adding information that isn't in the PNG file -- i.e., the converter would have to guess what lines and curves the bitmap is trying to suggest to the human eye. Maybe a sophisticated machine vision algorithm could do a conversion, but I bet it wouldn't look good.
Anyway, a piece set designed and supplied in SVG is obviously much better for scaling. If we have at least one nice looking SVG piece set to use as a default (do we? I haven't looked lately), dropping the ability to automatically select a pre-scaled bitmap set doesn't seem like a great loss to me.
By the way, the main set I used to supply was designed in Metafont, and I used to run Metafont to generate the various sized pieces, then do some hand-tuning of the bitmap. I wonder if Metafont (or some other program that understands the Metafont language) can render to SVG these days. It would be cool to have the old familiar piece shapes as SVG.