The solution for this objection would be to make selective sync a Plus or Premium feature for the desktop products. In fact, it would likely be a strong encouragement for convincing people to subscribe over using the free product.
Per my recent comment, I'm a Premium subscriber -- and frankly, I don't have that much stuff archived. I do have a very small had drive on my work laptop that I have no control over. If I have to start carrying a portable drive or stop using the Windows desktop app on my work laptop, I'll be better off using multiple free accounts or using a different product altogether.