I used to think like you, until I owned both the Prophet 6 (sold) and OB-6 (love and keeping for life!).
Sure, 5 octaves is generally always better (for us players). That said I've since found the smaller size has some good benefits and even looks in proportion with itself better.
I'm not saying a 5 octave wouldn't be ideal, maybe I'd upgrade? but I am saying right now it's not a deal breaker, and that's coming from a Piano player too who has the OB above an 88key piano controller (not midid up the oB is just fine as is).
There are some rare occasions I may need to go up top, but I just rethink it.. for studio anyway, if playing live? maybe.. but then you could just hook up a 5 octave controller or get the module instead. I like the 'cute' aspect of the P6/OB-6 sizing and it can be just as comfy on my ultimate stand, my desk (where 5 octaves wouldn't fit so well) or even on my lap on the sofa programming it. It's already quite heavy for its size (high quality and thick metal!) so it would certainly make it heavier. And I've had tons of heavier synths so am ok with them (JD-800 etc), I'm no wimp, but the OB-6 is kind of perfect as it is, even with its keys and lfo limits. It just kinda suits it. Also it keeps that price down.
Maybe buy a used one on ebay (plenty of good quality stock out there don't listen to horror stories SMH) you could save a large amount buying used then easily resell for the SAME PRICE losing nothing if sequential bring out a 5 octave VCO poly (which they probably will at some point though It wouldn't be just an OB-6 with 5 octaves I don't think but a new VCO poly entirely).