Presumably if the lounge becomes purely a BA Galleries lounge, it will be open to accommodate BA flights only. Currently BA has three flights a day out of Singapore: to Sydney departing 7.55pm and London at 10.55pm and 11.25pm.
If it's no longer a QF co-lounge it would stand to reason that the hours of the lounge would be designed to fit BA's schedule only. Assuming the opening time of 3pm remains, that would mean, as the timetable stands, QF flights to Sydney, Melbourne, Perth and Brisbane would fall into that window, although the midninight Singapore-Perth departure may revert to the morning which rules out access. This assumes no changes to the timetable but changes have been mooted.
And finally, what about oneworld Emeralds flying Jetstar? If it's purely a BA lounge, there goes access for Emeralds flying on Jetstar (eg. JQ8 to Melbourne, 3K111 to Perth, plus all the 3K intra-Asia flights that, currently, leave post-3pm. Given the indications that the Singapore schedules will be more aligned to meet connections with Jetstar, that'll mean less access again.
So I see this as, certainly, an improvement on the bad news but still not without some big losses for QF Platinums.