In my opinion, the strategy behind this particular change is actually a lot more clear than some of their past ones [1].
Allo didn't gain significant ground and failed to displace WhatsApp, so they can't fight Facebook on that front. Instead, they've conceded the phonenumber-as-indentity over-the-top-messenger market, and have pivoted to something more fundamental: the humble SMS app, and SMS itself.
They're baking all their Google Assistant hooks into the default Android SMS app, which will make it harder to avoid -- something Google definitely wants. This way, they don't have to fight Facebook and WeChat and Viber and Telegram on their competencies, because they've sidestepped them with the power of defaults for some amount of users, which will be a problem for the competition later on.