Setting up a team and tactics has everything to do with picking the best personnel to get results and manage them for the whole season. On the one hand you justify Amad's lack of game time with 'because ETH knows' but how do you explain playing a 19 year old Garnacho and a 18 year old Mainoo with no midfied in almost every game when available this season when they've clearly been tired and played way too much? Or Amrabat, who surprise surprise is not as shit as we'd all thought based on 5 min cameos here and there when he got two game starts in a row.
The whole basis of your doubt is basically 'because they're professionals and we don't know better', which is ludicrous. I'd imagine you've never criticised anybody regarding Utd then because that logic applies exactly the same. Of course we don't know for sure 100% what the reasons are but when you've got a whole season of results, line ups, substitutions, performances on the pitch and stats to evaluate then it's arguably way more informed than whatever you're suggesting.
He did exactly that in his appearance with the game winning goal against Liverpool. After his one match ban, he didn't play a single minute for another 3 games. As for the mentioned players, nobody is saying he should have replaced all or taken even the majority of their minutes (even perhaps Antony) but he should have had way more opportunities. That's not even mentioning how Forson came on and got more time than him from December.
As for merit, isn't that exactly what people are saying? If Rashford or Antony are not playing well, what do you know about what they are doing off the pitch that justifies them playing ahead of Amad? You don't. So to suggest people asking Amad to play (and in your opinion) not based on merit is a false equivalence. We can only judge what we see on the field.
As for your comment about people getting excited over a poor Newcastle, we've lost to worse teams and played even worse. What does that say about the merit of the players, who played in those games and still had dozens of games afterwards? It doesn't. It just proves ETH has been consistently bad with his tactics, picking of personnel and recognising the form/condition of his players, which is exactly the whole point of 'Amad should have played more'. It's really not rocket science or that controversial but some people are being so contrarian, it really boggles the mind.