I really don't see the argument for buying demand ceasing post...

I really don't see the argument for buying demand ceasing post the SPP - it doesn't make any sense to me. If there were to be any reduction in buying demand it should have been when the SPP was announced, as existing holders considering adding to their RAC holdings would have taken that money intended for on-market purchases and put it into the SPP instead. Anyone not holding RAC at the time the SPP was announced was excluded from participating, so buying since then has not been motivated by SPP participation.

Following the completion of the SPP, assuming that it is successful, we should hopefully see buying demand increase as RAC will be very well funded and it will be clear that buying on market will be the only way to get a seat at the table (for the foreseeable future at least). As you've said, this could also be supplemented by SPP participants buying more on-market in the event of a scale-back.