what you end up looking like.. is predetermined by your genetics. if you do everything as efficiently as possible... you will simply maximize your own genetic potential.. which is unique to everyone.
the mass you can build, maintain and lose.. the bodyfat you carry and are capable of burning... your structure and the shape and insertion points of your muscles.. are all genetically predetermined.
you are fixed. your structure is fixed.. all this talk of percentages... this is a game of physical appearance.. yes there are variables involved.. and if you work those variables perfectly.. your end result.. will still be - 100% determined by your genetics.
genetics... you can not escape them
If every variable is accounted for, and the most optimal strategy in every area = chosen, then, yes, in BB genetics will be the determining factor.
The only problem with your statement is that almost nobody come close to doing what it takes to get to this point, so while the argument = certainly has merit, it = not very applicable when discussing "building a good or great natural physique (Debussey assumes that we're discussing that here, not toptoptoplevel competiion)". In the real world, 97-98% of people that are into "building a good or great physique" never come close to putting in the effort, using the best strategies and doing it for long enough to ever max out their "genetics".
So the last line of your post should instead be like this (given that we're not only discussing the worlds absolute best bb's or whatnot): !genetics... you can not escape them... but youre gonna have a shitload of trouble maximizing them..."