I am sure that the 737 would be able to carry on with the flight and land safely at it's destination.However it was the captain's decision to go back to GVA since he was so close,something the BMI captain didn't do!
And one more thing for the drivers of the 737 that use the WX radar installed.
It is known that this radar antenna may depict very well the dimensions and intensity of a cell BUT...if another cell is present behind the depicted it tends to "underestimate" or even "omit" the presence of a second buildup.
I wouldn't judge on the cpt's actions with such ease !!
