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The goal isn't to completely replace PHP but to complement it. You can think of it like a playground for future PHP syntax. Something could be added to PXP and released in a week, PHP on the other hand has 1 big release a year. If something is successful and widely used in PXP, then it could be a contender for PHP core.
PHP is used on servers that can run many different languages. Additional there is only one php interpreter that is widely used (yes there was hhvm and some others ), that means you can rely on all features of this version.
The comparison just isn't relevant.