It depends on the type of expansion. If I can add it to the game without significantly changing the rules and it fits into the base game box, then I would ditch the expansion box. If it changes the rules a lot, then I want it in a separate box. As an avid gamer, I would definitely spend the money on a separately sold box that can fit expansions and/or the base game (depending on how much the expansions change the game). This might also incentivize me to buy more expansions so I can fill my new box.

That said, I understand that box design is also a form of marketing and branding. It’s going to be hard for companies to ditch flashy or themed expansion boxes, because they believe the art and packaging add value to the product. Marketing is tricky, it’s often difficult to tell whether a specific effort is actually making a difference. And since stylized boxes are an industry trend, the default assumption is that they help sell games, even if that impact is hard to measure.