Why? If you want PR, you can have that with a presidential system. See Brazil as an example. Americans have had presidential systems since before the country was founded (the colonial governments), have enjoyed virtually unparalleled stability with them, and show no eagerness to abandon them.

In fact, the UK, one of the oldest parliamentary democracies in the world and the originator of the "Westminster System" used in most Anglophone countries, moved towards presidentialism with the Local Government Act 2003 that allowed directly-elected mayors, such as London's colorful Boris Johnson.