How often are stream truncated at the start? In my career, I've seen plenty of end truncation, but start truncation never happens. Or, to be more precise, it only happens if previous decoding is already borked. If a previous decoding read too much data, then even UTF-8 is borked. You could be decoding UTF-8 from the bits of any follow-up data.

Even for pure text data, if a previous field was over-read (the only plausible way to have start-truncation), then you probably are decoding incorrect data from then on.

IOW, this upside is both ludicrously improbable and much more damning to the decoding than simply be able to skip a character.