There is no way that blesta is going to replace WHMCS.
WHMCS Is well known for alot of hosting company's.
Blesta, Well they not new but they do not have the rep like "WHMCS" has. Alot of people will continue to use WHMCS because there client and admin panel are well organized and easy to find things that you need to "for example" change things in timely manner
But hey that is my opinion other people may think differently