I often have heard Mahayana (and Mahayana-derived) Buddhists criticize Theravada for not having a Bodhisattva Vow — where one vows to be reborn again and again to teach sentient beings, until all sentient beings either have attained enlightenment or attain it together — accusing Theravadists of being selfish for trying to attain nirvana/nibbana quickly. Here are a few defenses of the Theravada view:
Westerners are often asked to take the Bodhisattva Vow shortly after they have attended a Mahayana meditation group for the first time, when they barely know anything about Buddhism, are nowhere close to enlightenment themselves (so probably could not control their rebirths), and are hardly in a position to make a long-term promise.
Like the Western elementary school lesson/game ‘telephone’ — where 20-30 kids sit in a circle, whisper the same message in each other’s ears going around the circle, and see how distorted the message becomes after it has gone around the circle — humans often misunderstand what they hear, and then they pass on that misinformation. Over time, the original message is lost. Buddhism has split many times, and later forms of Mahayana, Vajrayana, Pure Land, Zen, Nichiren, Tientai, etc. have become very different from early Buddhism, even in just a few thousand years. How different might human Buddhism become in 1 million years, if it survives that long? Would it bear any resemblance to what the Buddha taught?
Pop culture religion also has a habit of incorporating legends and becoming more outlandish/fantastical over time. For example, some people have apparently arbitrarily made up large numbers for the sizes, distances from Earth, or lifespans of beings in certain heavens and hells. I have also heard South Asian people claim that there have always been Indo-Aryan peoples in South Asia, though the archaeological record says that they have been there for only about 5,000 years. Some people claim that Islam has always been in India, though Muhammad (peace be upon him) lived from 570-632 CE.
There also is the issue of how much karma Mahayanists must keep in their mindstreams, in order to remain in samsara. Some of them, especially Zen Buddhists, lead quite worldly, indulgent lives. It is unclear whether they are clear-headed enough to preserve and practice good-quality Buddhism.
If there are many, possibly infinite, fully enlightened Buddhas from the distant past living forever in Pure Lands, which they created for themselves and which samsaric beings can visit, why do ignorant humans, or even devas (long-lived, heavenly beings), need to take a bodhisattva vow? Won’t everyone encounter one of those Pure Lands, or beings who have visited those Pure Lands, eventually? If there is an infinite series of past Buddhas, why don’t they continuously come to Earth themselves, or create real-time projections of themselves on Earth with which people here could easily interact? If Buddhas gain Creator God-like power over Buddha Nature when they become enlightened, why are their powers to interact with humans apparently limited after their human body dies?
(For how long) Would a Mahayanist sit around waiting for solids, liquids, gases, etc. in the samsaric universe to possibly evolve into a sentient being that is complex enough to interact with and instruct on the path to nirvana? It has been about 13.8 billion years since the Big Bang, and there is a great deal of matter in this universe that is nowhere close to evolving into complex life, and may never be. Physicists’ predictions about the eventual fate of the universe (trillions of years from now) describe a great deal of matter either never evolving into sentient life or being destroyed in a Big Crunch. Upon what nearly-permanent bodymind medium do Mahayanists plan on surviving until the end of the universe, or across universes if there are multiple Big Bang – Big Crunch – Big Bang… cycles?
It is very rare and transient for sentient life to evolve in this universe, and it might happen on worlds separated by vast distances. How do Mahayanists plan on traveling to such worlds to teach the people there?
As Thanissaro Bhikkhu said in one of his recorded dhamma talks: “This body requires that we have to take food, clothing, shelter, medicine…. And it’s not only a burden for us in the searching, but it’s a burden for other people in their providing… other beings of all kinds: animals. This is why, when we stop samsara-ing, it’s a gift — not only to ourselves, but to the people around us. It’s not selfish to stop doing this. If you thought of samsara as a place where people are suffering, then it might seem heartless to want to get out. But, if you see it as a process — a process that’s causing yourself suffering, a process that’s causing other people suffering — the more people who stop doing the process, the better everybody’s going to be… the happier everyone’s going to be” (“Constellations of Stress,” 2004-09-07).