I'm also not seeing what makes Beto unqualified, particularly in comparison with the rest of the potential 2020 field. Every potential candidate has downsides. Biden, too old and centrist. Warren, too old, liberal, and unfairly has to deal with a Hillary level of misogyny. Harris, too young, liberal, and inexperienced. Booker, too corporatist and contrived. Etc.... These are not my criticisms as I think any of these would be great presidents. Just what I anticipate voters will impute onto the candidates.
Beto has downsides to be sure, but in my mind they are far fewer than the remainder of the field. Most importantly, he has Obama level charisma and is an excellent and very experienced campaigner and fundraiser. He's progressive but actively seeks to bring disparate views together rather than just being a liberal bomb thrower. And while it's not fair, the fact that he's a white man would probably make it easier to appeal to swing state Obama voters that went for Trump in 16.
Any number of candidates could beat Trump, but Beto at this point appears to be the strongest potential candidate so far. Of course a lot could change over the next 18 months and a relatively unknown governor or other unknown candidate could emerge as a similarly strong option.