There cannot be feudalism without the narrative of an omnipotent god to justify class inequality.
There needs to be a strong narrative to justify why some people get free stuff and others don't.
And now you are making a gross mischaracterisation of nirvana. It is nothing like Heaven (Buddhism has heavens and hells too, or higher and lower planes of existence). And no, nirvana does not 'serve the same purpose as a god'.
If the class difference is big, the narrative to justify the difference must be strong; even under serfdom or slavery, there were uprisings when the narrative was weak. The lowest class has to be (to some degree) content with their situation.