When the Temple in Jerusalem is rebuilt, won't the sacrificial services be restored and won't people have to eat meat?
As indicated previously, Rav Kook and others believe that in the Messianic epoch, human conduct will have improved to such a degree that animal sacrifices will not be necessary to atone for sins. There will only be non-animal sacrifices to express thanks to God.
As also indicated, based on the prophecy of Isaiah (11:6-9), Rav Kook and others believe that the Messianic period will be vegetarian.
While most Jewish scholars assume that all Jews ate meat during the time that the Temple stood, it is significant that some (Tosafot, Yoma 3a, and Rabbenu Nissim, Sukkah 42b) assert that even during the Temple period it was not an absolute requirement to eat meat! Rabbenu Nissim characterizes the "requirement" to eat the meat of festival offerings as mitsvah min ha-muvhar, that is the optimum way of fulfilling the mitzvah of rejoicing on the festival, but not an absolute requirement. Moshe Halevi Steinberg, in the responsa previously mentioned points out that vegetarianism for health reasons did not conflict with halacha even in Temple times. He indicates that one could be a vegetarian the whole year, and by eating a kazayit (olive-size portion which, due to its size, would not damage his health) of meat, he would fulfill the mitzva of eating the meat of sacrifices. Even a kohen (priest) could be vegetarian except when his turn came to eat of the sacrifices during his period of duty (about 2 weeks), when he, too, could eat just a kazayit. He actually could eat even less according to the Hatani Sofer, since many kohanim could join together to eat the required amount, so that the vegetarian kohen could eat even less than a kazayit.
R. Steinberg notes that among the things listed as disqualifying a kohen from service in the Temple, vegetarianism is not included, since he could arrange the problem of the eating of the sacrifices in one of the ways listed above. However, R. Steinberg adds, a kohen who became a vegetarian because his soul recoiled against eating meat would not have been allowed to serve in the sanctuary since if he forced himself to swallow a kazayit of meat, it would not fulfill the halachic definition of "eating".