If maintaining the current functionality of Healing Word in a system without Bonus Actions was the goal, it could easily be done. Just include in the spell description, “When you use your Action to cast this spell, you can use another Action on your turn. This Action cannot be used to cast another spell, other than a Cantrip.”
It’s entirely possible to exactly reproduce every Bonus Action effect in 5th Edition as an Action that allows you to do something else on your turn as well. You just have to specifically call our in the ability what else it allows you to do.
So now a creature can cast healing word, Dash with Cunning Action, perform any other number of "half-actions" that allow other actions to be taken, and perform any Action that has not become restricted by the "half-actions" taken on the turn. A creatures turn can turn into a five-minute affair (yes, it's hyperbole) while everyone waits for all the Actions to be completed.
Or we can have bonus actions which cleanly consolidate things down to 1 bonus action a turn. While every bonus action could be rewritten as an action, the extra column inches this would take would take pages over the course of a book. Additionally, in certain cases, I think it would make a worse option than as a bonus action. In this case, I think the designers made the better choice already in the PHB rather than what Mearls is bandying about now. On the other hand, I appreciate that he talks about what he is thinking so that it gets out into public discussion.
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