The cost could be countered by the possibility of it lowering over the gen. Eat it now and reap the benefits later.
It probably is unlikely. I just don't think we can definitively rule it out. Things have already happened that only a few expected to happen.
Not to mention that, as has been stated so many times, the rumour is not that PS5 will have HBM2 instead of GDDR6. The HBM2 is only supposed to make up a third of the RAM. The rest is DDR4, which I believe is considerably cheaper than GDDR6 (how much cheaper is it, anyway?). If we then take into account what the rumour said about buying cheaper rejected chips and using InFO_MS to improve those chips and avoid the interposer cost, along with what you said about the HBM2 dropping in price quicker over time, would this solution really cost any more than 24GB of GDDR6? And if it did, would the cost be a huge issue given the space savings and the extra power that could be allocated to the GPU?