How and when to decommission your on-premises Exchange servers in a hybrid deployment

So the reason why you'd want to keep at least one Exchange box is for management purposes, as mentioned above. Microsoft only supports managing Exchange related objects and attributes via the Exchange management tools, no way around it. If you are fine with being in unsupported configuration and feel comfortable performing those management tasks via PowerShell, ADUC, ADSI Edit or whichever tool you prefer, you can certainly go ahead and remove the last Exchange server too.