I don't see anything wrong in spending to create a competitive team.
Certainly nothing wrong with it in theory. Besides, Napoli can risk 1-2 big players flopping and even going out of the CL. For years they have been a nothing team, so it would hurt, but finishing 4th or below would not be anything new for them. They'd have to sell a big player or two but they'd have another go. Juventus can't do that. Out of the CL and it is a disaster from a sporting perspective.
ADL is a fan firstly, then president. His moves are very much like what a fan would do if you gave them a €100m budget. Napoli have never really had this money to work with and came from absolute zero, so he is just throwing bids everywhere, just like that kid in the sweet shop.
This makes them a very dangerous prospect for us as a challenger, in the short term at least. They'll very much go for glory, bang or bust. I'd expect Juventus will be much more steady over the years, but we won't be able to really judge until further down the line.
Personally I'd rather ensure my club was stable. Look at Inter, that is a classic car crash example.