Of course Nadal wants to play it on clay because he's almost unbeatable on clay, he never really says lets play it on grass as well (at least I can't recall him saying that). I think first it makes no sense to change surface for one event, also indoor deserves a big event. Would Nadal prefer if the WTF had been rotated and the AO had always been on an indoor court?
WTF is indoors, and presents a challenge in the same way other events which don't change their surface do. Nadal has always met these challenges and really it's not beyond him to do so again. I wish he'd just relish the challenge like he's always done because that's one of the things I most admire about him. Call him a dirtballer? He wins Wimbledon. Say he can't win on HC? Wins AO beating Federer there. Say he can't win the USO? Wins USO. Say it was a fluke? Wins it again. Say he hasn't won a HC title in years? He wins the US Open. Again.
With the wtf there's been Federer and Djokovic there winning multiple titles but Nadal has beaten both of them at the event. There is no reason he can't beat anyone there on that surface, it's just not 99% in his favour like most clay events are (though I don't think it would be as easy as MC or RG) By contrast Federer had the almost unbeatable Nadal to contend with at RG, but I doubt Nadal would have been happy to make RG a grass or indoor HC event for even one year in the last 12, just to make it "fair"
Another interesting thought, that if you did swap between HC, grass and clay, given the amount of events he's missed it's possible he'd still not have won it. In the years Nadal has competed let's say it was
2006 - Grass,
2007 - HC
2008 - Clay but Nadal did not play that year
2009 - Grass
2010 - HC
2011 - Clay (but vs Djokovic that year who had beaten him twice on clay in straight sets, not the favourite...)
2012 - Grass, didn't play
2013 - HC
2014 - Clay but didn't play
2015 - Grass
2016 - HC but didn't play
maybe this year would be his year. Or would he have played in 2014 and 2008 if it were on clay?