Being liberal in acceptance did not discount the conservatism in output. If diligently only one side of the maxim is followed, there would be less issues. For example, if many implementations were conservative in sending, that alone is sufficient. After that allowing for liberal handling of input is to compensate for those who did not follow the conservatism in output. Postel can't be easily blamed for long term entropic outcomes, accelerated by the abundance of implementations that ignore both sides of the maxim.