Isn't that putting all eggs in same basket?

"Engineered knockout or deletion of the E gene has effects ranging from moderate124 to severe293,296 to lethal.105,428 Thus, although E is not always essential, it is critical for coronavirus infectivity"  

Fields Virology 6th edition

"Data shows that E is involved in critical aspects of the viral life cycle and that CoVs lacking E make promising vaccine candidates"

Virology Journal     Published: 27 May 2019

Coronavirus envelope protein: current knowledge