What's the downside for bonds here?

That the Fed might raise rates? Fat chance
That treasury yields might rise significantly from here? The Fed will act

Literally the only risk is that you're missing out on a further bull run in equities, when we are already at Shiller P/E 38.

This market literally has a backstop.

>start saving and investing
>line go up
>finally, a life plan

>suddenly feel the urge to pick up more hobbies
>suddenly feel the urge to have kids