Updated parent inheritance

[koenner01]

Fix for #8587

[maghamed]

replied in the issue #8587 (comment)

@koenner01 Why do you need Image to be successor of DefaultFrontend?

An example of where this is already done in the core:
Magento\Eav\Model\Entity\Attribute\Frontend\Datetime

We don't have these neither in 2.0.*
https://github.com/magento/magento2/blob/2.0.13/app/code/Magento/Eav/Model/Entity/Attribute/Frontend/Datetime.php#L11
nor in 2.1.*
https://github.com/magento/magento2/blob/2.1.4/app/code/Magento/Eav/Model/Entity/Attribute/Frontend/Datetime.php#L11
nor in develop branch
https://github.com/magento/magento2/blob/develop/app/code/Magento/Eav/Model/Entity/Attribute/Frontend/Datetime.php#L11

Actually for now Magento\Eav\Model\Entity\Attribute\Frontend\DefaultFrontend class doesn't have any successor.

Currently the only place where Default Frontend class is used is in the AbstractAttribute class

    /**
     * Retrieve frontend instance
     *
     * @return \Magento\Eav\Model\Entity\Attribute\Frontend\AbstractFrontend
     */
    public function getFrontend()
    {
        if (empty($this->_frontend)) {
            if (!$this->getFrontendModel()) {
                $this->setFrontendModel($this->_getDefaultFrontendModel());
            }
            $this->_frontend = $this->_universalFactory->create($this->getFrontendModel())->setAttribute($this);
        }

        return $this->_frontend;
    }

and this behavior looks correct for me.
What customization abilities you are losing, having such hierarchy of inheritance ?

Magento discourages usage of inheritance. So, a deep inheritance tree is a "smell" from architectural point of view.