Fangraphs and BBref are pretty different on this. Didi is a mere 0.2 fWAR, so basically a wash, but Simmons is 2.0 defensively to Didi's 0.6.
BBRef WAR, Simmons is 0.9 to Didi's -0.4, which could be worth a full win - and the difference in the NL East - if that played out the same way over the last two months.
I think a better defender there might help the team especially when Didi isn't hitting either, and we have a team full of hitters. But the bench may already be providing enough, and the other factors (giving up something, Didi's salary) make a trade unlikely.
That said, the Phillies probably signed the wrong guy. They'd be no better or worse, but without the 2022 obligation to a declining player. And Galvis is better than both of them.