When we look more closely at what postmodernism is actually post- or against , we see that monism is closer to (but is not exactly) the concept that postmodernist oppose.
1950s style American mono-culturalism versus 1990s multi-culturalism.
1950s Normal families and Deviants versus today's Alternative life-styles.
I use monism as a stand-in for meta narrative , in my understanding of Lyotard's sense of that word:
that there is only one truth on any matter, that it is easily knowable ( and easily known to be the one and only truth) --- and that all other claims to be part of that truth are in fact inverts, deviants, unfits, weaks, simples, degenerates, deficients, defectives and either have to be corrected or destroyed.For one example,think of the 1880s-1980s orthodoxy that medical scientists should only study free floating "P"lanktonic "S" or "M" virile S. pneumococcus bacteria--- not their harmless (and hence useless) kissing cousins, the "R","L" and "B"iofilmic" S. pneumococcus bacteria.
Dawson bucked this orthodoxy his whole career ---- and we have DNA and penicillin as a result....