BT's vote doesn't guarantee you a top 20 spot. Did they get endorsed because they were close to becoming top 20 or were they already a top 20 witness?
Could someone become a top 20 witness without BT's vote? Absolutely.
Would it be harder to achieve without BT's vote? Again absolutely.
The problem is not enough people vote for witnesses or if they do vote they just vote for whoever is in the top 20 and call it a day.
Another problem is that there are a ton of people who don't care about governance and just only care about earning. Whatever small amount of HIVE they receive is powered down and sold.
I haven't really seen BT powering down.