Ok,
the field is not disabled ... it is readonly
so if you allow it to be required ... then Joomla default profile form

-- will succeed to save properly

but
then any custom template overrides that are now skipping the username field
(skipping when !JComponentHelper::getParams('com_users')->get('change_login_name'))

-- will fail to save
(i do not know how common this is or even if they hide it instead of skipping it, in which case saving will succeed)

I do not remember if there was a bug report for the above in the past
but i think there was

Also making the field both disabled and required
will again make the form to fail to save always

so i am afraid the
$form->setFieldAttribute('username', 'required', 'false');
inside model's code is deliberate ?

Maybe we need to make a fix / improvement inside JForm validation
to avoid failure when a field is both disabled and required

Also if there is custom code that is using a model instance to save the profile, and is not providing username when JComponentHelper::getParams('com_users')->get('change_login_name') is OFF will fail

a simple fix is to patch the form template file ...

<?php if (!$field->required && $field->type !== 'Spacer') : ?>
	<span class="optional">
		<?php echo JText::_('COM_USERS_OPTIONAL'); ?>
	</span>
<?php endif; ?>

i mean maybe it makes sense not add optional / required markings when a field

• disabled or readonly