Mateta hasn't done anything wrong. He has worked his way up to the edge of the highest level, nothing has been given to him. He has more than repaid Palace for his fee and wages, both in terms of goals, performances, and attitude.
He cannot achieve much more at Palace. We have been on the crest of a wave but that has now inevitably fizzled out. He has seen his better teammates leave for higher things, and is entitled to desire the same for himself. In fact it would be odd, and a bit concerning, if he didn't want that for himself. We chose to set up as a 'buy, develop, sell' club. Mateta has been bought, and has developed. Of course he now wishes to be sold.
He is now getting to 30, and what will probably be his one and only chance to play for a famous historic club and try and win titles. He won't be first choice at a top premier league club, or a champions league contender club, but could be first choice no9 at a faded Italian giant who will contest for the league title. Given his age I imagine the big Italian clubs probably still possess that alure, which perhaps they don't for a 20 year old. I doubt it has much to do with money - he could make loads staying at Palace. He wants to fulfil his dreams.
He did the right thing by Palace, telling the club nice and early that he didn't plan to extend his contract so that Parish and Hobbs could make informed decisions. He didn't string anyone along. He has played on with an injury for some time, and returned to play with it. OK, he might have had an eye on the World Cup, but his other eye would surely have been on the consequences for his family's future if the knee deteriorated or he picked up another injury through playing. Any move would be off, and lets not kid ourselves that Palace would still offer him a long term contract if he got injured. None of this was obvious in his play. He did his job fully, and well.
I have heard that Italian medicals are more stringent than premier league ones (hence Khalaili failing one at Inter but signing here). Perhaps that will scupper any sale or even free transfer move in that direction, and lead Mateta to sign a new contract here. In a way, that would be a shame. I'm not sure we would get the same player over the next four years as over the previous two or three. Partly he would be older, and is already a big, heavy bloke with a bad knee. Mostly, though, he would be a man whose dream had been dashed and had made the pragmatic choice instead. That's what we had with Benteke, Sahko, etc.