For starters, it's not confirmed they are wrestling on PPV whatsoever. That's merely a conclusion fans have drawn up. Could it happen ? Yes. Will it ? That's the question...if they stick to the details that have been stated in this storyline, the answer would likely be no. Ryan has openly talked about PPV paydays in one of his promos and on twitter, thus it would make little sense to put a guy who isn't under contract, has mentioned the paydays and whom Snow says cannot and does not draw, on PPV.
The match also won't be a world beater, so using it to pop a TV rating IMO would be smarter, as it likely won't pop a PPV buyrate. But there is a chance especially if it happens at the top of the 2nd hour they can pop the TV rating for that quarter.
As far as Taz being replaced, not sure what you are trying to say but Taz was never going to wrestle Ryan, unless the call would be for a 30 second match where Joey essentially gets locked into a Tazmission and that's it. Taz cannot wrestle anymore and will not wrestle anymore. Maybe I misread what you meant but if I did not then I present this...from back when Taz debuted with TNA he posted this on his FB page :
Hopefully for anyone who thought Taz was originally going to wrestle, this clears that up and they realize the storyline was always meant to lead to Snow vs Ryan. Taz was used as to bridge between and allow them more time to drag the story out and slow burn it. Taz/Ryan's on screen confrontation also made for great freaking TV.