It seems to me that a "take it or leave it" ultimatum only makes sense if you have a specific alternate course of action that you intend to follow if the answer is "leave it" - and particularly if that alternative is somehow time-constrained.
I don't think the Phillies have such an alternative in mind. Yeah, I've seen the stuff about Keuchel/Kimbrel, but I don't think it's real. Keuchel just doesn't impress me as worth the risk - both that he's not as good as he appeared a couple of years ago, and the inherent risk that a pitcher will implode physically. And Kimbrel? Jonathan Papelbon all over again. The Phillies' season isn't going to turn on the difference between having Kimbrel and instead relying on Robertson, Dominguez, etc. Improving an already-good bullpen corps isn't going to make any difference if the club isn't ahead on the scoreboard come the seventh inning.
My guess is that the Phils' alternative is to go with what they have - if they can't land a clear improvement (Harper), there's no point in grabbing a middling/maybe improvement - it's just as likely that the moves they've already made will work as would pulling in another "minor" improvement that could as easily bust.
I do not think an ultimatum presented simply because the front office "wants to get it over with" is wise. If they want Harper, they'll let this play out. An ultimatum, which could easily push the decision the other way, is the kind of strategy you employ if you want to appear tough, if you want the fans to think "you really tried" - if you're more interested in appearances than outcomes. I kinda hope that's not what's motivating Middleton.