I agree that there are lots of ways to improve the playback and that more such dedicated samples could clearly add realism :). Yet, I think the automatic selection you are asking for would be hard to completely implement in a sample player, since as you write a human player looks ahead and knows that a particular note is a breve, but the sample player (Synchron or Vienna Instruments pro) inherently cannot. Something like this would have to be implemented in the DAW or Notation Software, which knows how long a given note is and in which context it appears. E.g. Steinberg's Dorico already allows to automatically choose the playback depending on the note length. I am not entirely sure about Synchron libraries, but full Vienna Instruments series libraries actually already offer up to four different legato versions (slow, medium, fast and the performance trill at the highest speeds) as well as a progressive vibrato legato version you mentioned, that can be used for such very long notes.
In case you are a user of Vienna Instruments series libraries, we have implemented a similar automatic selection you are suggesting in Articulate Presets (based on VI pro's speed control, which is all that can be done at the sample player level and which uses the previous notes to predict following notes). The Cello, in particular, is included in our free demo package, so if you are by chance a user of the VI Symphonic Cube or the VI Solo Strings library, you can simply download it and check out if this is similar to what you are looking for (Articulate Presets are large and the corresponding speed-controlled legato is matrix #24, whereas the legato with progressive vibrato is #14). The automatic selection based on the note length is likewise already implemented in the Articulate Map for Dorico, which uses the current maximum of 5 versions for different note lengths, but even Dorico's selection mechanism does not allow developers to define a dedicated playback for a breve so far ... hopefully this will improve soon.