I do like smaller kit but making lenses smaller usually involves compromises. I haven't seen any pictures from this lens and I only have the M mount to go on plus my experiences with the modern and film era lenses I have but I'd guess that making this lens a pancake or even just making it smaller than this one would impact on its optical qualities to the point that they'd become more obvious and would lead to negative comments in reviews and that could hit sales even if the issues are only more obvious wide open to f2.x or so as with their existing small Sony mount 35mm f1.4.
I'm all for smaller kit but I have I think very few people reading this would accept the image quality especially at wider apertures. One good example of this (and ignoring the new make far eastern lenses like the Pergear and Artralab for a moment) is that E mount Voigtlander 35mm f1.4. Its performance at f1.4 has obvious flaws and people will avoid it for that reason ignoring its performance stopped down. I'd imagine that a similar sized 28mm with similar flaws would be an even more niche seller than this larger and probably better performing but larger lens. Of course Voigtlander could have avoided some of the performance hitting difficulties and made a smaller 28mm f2.8 but do people want that these days? I think f1.5 is even stretching it, I think people would much rather see f1.4 at a minimum even if that affects size and price. The fact that I'd probably accept a smaller f1.4 with issues means nothing as I'll be in a minority within a MF minority. I'm pretty sure that the Voigtlander 35mm f1.2 was a better seller than the f1.4 despite the additional size and weight but I could be wrong.
I am a bit surprised by this new 28mm f1.5. I thought they might do a 28mm similar in size and weight to their 35 and 50mm f1.2's but for whatever reason it looks like they're aiming between their 35mm f1.4 and the f1.2 lenses maybe based on what they thought people wanted for the M mount lens.
I'll have a look at this lens when it hits the reviews and forums and I might buy one but having said that I've not done much picture taking in the last couple of months so I really shouldn't put more money into kit that'll hardly get used... but then again... You're only here once