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.
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.