Fermentation penicillin ( aka natural, biological penicillin) worked (feebly) from day one (September 1928) but could be (and obviously was !) improved gradually.
By contrast, even after 15 years, synthetic penicillin in late 1943 still hadn't been gotten to work at all. (IE, biologically : all resulting attempts showed no medical activity at all.)
In a way , synthetic molecules are completely like pregnancy.
As with pregnancy, there is no half-way house : you are or you aren't ; a synthetic molecule either works, or it doesn't.
In addition, even if synthetic penicillin could be gotten to work biologically, there were still two possible huge problems ahead of breaking out the champagne.
The yield could be much lower than the already low yield of Fleming's original strain of penicillium, not many magnitudes higher.
And the number of steps required in the synthesis and subsequent purification/ separation might require more, not less, money, equipment, manpower, care and close attention to detail than even the worst version of the fermentation method ever called for.
I have one word for you my boy. Rosebud ? No : quinine.
These are not just theoretical objections : as bad luck happens they all came to pass with synthetic penicillin.
The pursuit of synthetic quinine , now a more than two hundred year long futile chasing of a tail by generation after generation of chemists , should have reminded the 1940s penicillin synthetic faithful of the possible pitfalls that could lie ahead......