BG3 was not possible without dnd 5e. WotC would have never allowed BG3 without the current rules set.
But Larian has its own system already and also already voiced displeasure with hiw restricting DnD is. So its very doubtful that they continue with it.
What made BG3 so successful was neither 5E or even the Forgotten Realms name, but being a successor to Baldurs Gate and a sex centric marketing.
