It would've helped their chances at doing this if they integrated the features that make other messaging apps so strong. Until then, I don't know if Allo's features alone merit such an optimistic prediction. Why do you think it'll succeed in its current incarnation?
Or maybe you meant eventually as in after they've added these missing features (SMS, desktop continuation, etc)?
Just being an already installed system app and based off the users phone number is the major reasons. Added with the features already shown.
Both Allo and Duo which are integrated together and system wide, so it's a no brainer it will be widely used, even if SMS isn't integrated into it yet. The majority of people use Android, so you'll have plenty of users just using Allo and Duo with other Android users. And Duo is also available on iOS, so it'll be even more widespread of usage.
Hangouts was too robust and buggy for users. Separate dedicated apps that integrate with each other is much more inviting to users. And support for the apps can be more focused.
All I said above is the reason why I think Allo along with Duo will succeed.
IMO ... Maybe Google is playing it safe and will add SMS integration shortly after. I think Google put to much effort into Allo and Duo for it to fail.