Part of the problem lies with Arteta's historial reluctance to widen the overall experience of the squad, hence in this type of scenario he can't turn to, for example, Kabia or Butler-Oyedeji instead of Saka et al because he wouldn't even give them minutes when we were 7 goals to the good against PSV, so if he wants to change a game he's stuck with the usual suspects.
But I'm surprised you complain at his bringing them on given you also express disappointment at not winning the game.
I didn't like Partey at RB (for it was there that he was played, not LB...) but we're slightly stuck for alternatives given the above plus injuries.
That's why for me it's playing Martinelli that stands out - we have an obvious alternative in Sterling yet he wasn't used to rest Martinelli, who instead nearly had his leg broken in what should have been a red-card tackle (and would no doubt have been if it had been the other way round).