I honestly don't know how well Beto will do. He's doing the Obama thing of sounding progressive, but not really backing it up. He's a great speaker.
I think, in the end, he will take mostly from Biden. I feel like he'll grab mainly older people who are looking for "compromise" and for congress to "get along." Millenials are stuck on Bernie, and won't budge, IMO. Bernie's got a pretty decent floor that will stay. I'm interested to see how this primary will go. I can see a lot of different scenarios.
I'm mostly worried for the scenario where Biden wins. A lot of people seem to think he'll crash and burn. I disagree. I think a big chunk of the party doesn't care about his past. A lot of Dems are (at least a little) racist and misogynists, and will like Biden. They don't need to represent a large part. Just 25-30%.
Beto running might be good, then, if he can siphon enough votes away from Biden, to give the race to Bernie or whoever might be second.
Who knows. We are still at the very beginning. A lot can happen/change.