I assume that if this is a serum that comes only from bovine fetuses, more cows are slaughtered per pound of cultured meat than per pound of natural-grown meat. That would make this much less sustainable.
FSB is a byproduct of existing slaughter, not a special process which kills extra aninals. If you're opposed to the use of FSB because you thinks it's immoral to kill the animals, you should be equally opposed to the meat and dairy industries as they are today. Both are responsible for the death of animals and often fetuses.
There is a risk that the FSB collection process will allow the fetus to gain consciousness before death, but those paying into the meat and dairy industries are already comfortable with killing conscious, sentient animals.
It seems that you are thinking a little too short-term here. We have been culturing cells for >50 years. If this is a necessary ingredient, it may be a lot harder to substitute than you might think, which relegates lab-grown meat to staying a niche product. You will not have a viable, scalable meat replacement if your replacement needs to slaughter more cows per kilogram than meat.
As long as the amount of cultured meat is _relatively_ small, it's sustainable; FBS is essentially a byproduct, and no-one is slaughtering extra cows just to produce it.
If the cow was going to be slaughtered anyway, and you still get the meat, what does it matter if you get less serum per cow than meat per cow? Shouldn't we be glad we're getting more utility per cow slaughtered?
There is a risk that the FSB collection process will allow the fetus to gain consciousness before death, but those paying into the meat and dairy industries are already comfortable with killing conscious, sentient animals.