I would guess that there is not much room for them to develop the battery technology further, as it seems they are close to the absolute limit of the materials'

I would guess that there is not much room for them to develop the battery technology further, as it seems they are close to the absolute limit of the materials' performance. So to start a plug-in hybrid program using that battery technology, could be very bad (outpriced, outperformed) if there was suddenly a huge leap forward in Li density and safety.