You mean they need WotC's expertise to handle D&D 5E properly? Or to make a good game?

As far as the former... I think that the partnership was a major factor in BG3's success, but I expect it has more to do with the D&D brand and BG nostalgia, than any virtues of the 5E system. Maybe WotC's contributions to worldbuilding and lore helped... Larian are of course good at that in their own right, but there's a whole Forgotten Realms canon to navigate. (I don't actually know what WotC contributed in that regard, mind you)

In the case of the latter... The Divinity system is pretty heckin good, and in many ways a better CRPG system than any edition of D&D. Larian ARE experts at making really solid CRPGs, after all. The Divinity series is perhaps the most successful ever, maybe now behind BG3... So returning to their own IP would not be shooting themselves in the foot by any stretch, IMO. More like trading one kind of overwhelming success for a different kind of overwhelming success.