An outlier case, which doesn't justify changing the setup and introducing even bigger issues/abuses to solve.

If accounts allowed transfers, I'd personally have no issue with doing something like:

• Have my own personal "main" steam account
• Use secondary accounts to buy games
• Transfer any game I want to play longer term / figure I'll revisit over to my main account; sell the secondary account on Facebook Marketplace/similar sites to recoop funds for myself on a quarterly/annual basis (removing that potential revenue from steam)

That sort of abuse would be far more common, than the outlier case you've listed for a young parent passing down games to a kid that's fairly close in age.