It seems like you don’t watch full Arsenal games all that much (which is perfectly understandable). Havertz was fine in midfield for us. He won a crazy amount of individual duels, never stopped running and had better defensive stats than Xhaka in his last season with us. However, he wasn’t as productive as Xhaka was in his last season so the were doubts over whether he was worth the price tag.
He’s never been garbage and was very much a competent midfielder. He's been a near ever-present in the team that is currently top of the league and prepping for a CL QF, he’s been more than competent - he’s been a real asset.
I dont watch Chelsea all that regularly, so if you think he’s no different to the player you had for 3 years, I’ll take your word for it. But he’s currently one of the best players in a team that is challenging for the most important trophies, so if that’s actually true I don’t think it was a good move to sell him.