Salesforce Lightning: Setting a conditional default value blocks any further changes to the attribute. Including object will allow changes but only from the object.

TestComponent.js

<aura:component implements="flexipage:availableForAllPageTypes">
    <!-- Simple example of conditional default blocking changes -->
    <aura:attribute name="myString" type="string" default="{!if(false, 'Default Val', 'Other Default')}"/>
    <aura:attribute name="mySimpleString" type="string" default="Default Val"/>
    
    <!-- Conditional default on object value will update, but will not allow external changes -->
    <aura:attribute name="myObj" type="object"/>
    <aura:attribute name="myObjString" type="string" default="{!if(empty(v.myObj), 'Default Val', v.myObj.val)}"/>
    
    <aura:handler name="init" value="{!this}" action="{!c.doInit}" />
    <div class="slds-card">
        {!v.myString} <!-- Will always output 'Other Default', even if changed later by doInit -->
        {!v.mySimpleString} <!-- Will be changed to 'Set Directly' by doInit -->
        {!v.myObjString} <!-- Will be changed to the value of v.myObj.val when it changes -->
    </div>
</aura:component>

TestComponentController.js

({
	doInit : function(component, event, helper) {
                component.set('v.myString', 'Set Directly');
                component.set('v.mySimpleString', 'Set Directly');
        
                component.set('v.myObj', {val: 'Set in Object'});
                component.set('v.myObjString', 'Set Directly');
	}
})