The Italian League if it wasn't horribly corrupt would have something to say about it, as it's unnecessarily weakening one team. Couple of teams got into trouble a few years back here when they fielded weakened teams, it's not much different.
This is part of what I was talking about back in January when I wrote about conflict of interests with owning multiple clubs. I agree that if the Pozzos want to move players on permanent deals, it will be easy for them (assuming the players are willing, a lot of this relies on player good will). But if Watford go up, will they prioritise talent to Watford, Granada or Udinese (the club they really love)? If Watford stay down, their best players will now be match fit and ready for Serie A or even La Liga. In will come a new influx of young and has-been Italians and Spaniards, denying Watford the continuity that was the only thing that prevented them getting automatic this year. The Pozzos only see Watford and Granada as a way to bring in promising, young talents from England and Spain, mainly to make a profit on. Their MO is buy cheap and sell high... but definitely sell. Of course it will be beneficial to them (and Udinese) to have Watford and Grenada in the top flight but they won't invest heavy to do this (they don't buy expensive players). Nor will they loan/sell them the best Udinese have to offer. The best Watford can hope for, as we saw with Grenada last season and this, is survival by the narrowest margins. I realise, by Watford's standards, that this must sound great to them right now, but a few years of relegation battles in the Prem with no real hope of pushing on to, say, levels reached by Swansea, Stoke, West Brom or Fulham will be frustrating. Meanwhile, any good young talent their academy produces will be (eventually) sold, with the money going into the big Pozzo pot as opposed to Watford, or they will be shipped off to Udinese.