Magento – Set default value for product attribute in magento 2.1

I have Create An Module For This Purpose. It May be help full for you.

 app\etc\modules\Designo_Productname.xml

<?xml version="1.0"?>
    <config>
        <modules>
            <Designo_Productname>
                <active>true</active>
                <codePool>local</codePool>
                <version>0.1.0</version>
            </Designo_Productname>
        </modules>
</config>

app\code\local\Designo\Productname\etc\config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Designo_Productname>
             <version>0.1.0</version>
        </Designo_Productname>
    </modules>
    <global>
        <helpers>
            <productname>
                <class>Designo_Productname_Helper</class>
            </productname>
        </helpers>
    </global>
    <admin>
        <routers>
             <adminhtml>
                  <args>
                      <modules>
                          <Designo_Productname before="Mage_Adminhtml">Designo_Productname_Adminhtml</Designo_Productname>
                      </modules>
                 </args>
            </adminhtml>
         </routers>
     </admin>
  </config> 

 app\code\local\Designo\Productname\Helper\Data.php

<?php
 class Designo_Productname_Helper_Data extends Mage_Core_Helper_Abstract
 {
 }

app\code\local\Designo\Productname\controllers\Adminhtml\Catalog\ProductController.php

 <?php
 require_once 'Mage/Adminhtml/controllers/Catalog/ProductController.php';
 class Designo_Productname_Adminhtml_Catalog_ProductController extends Mage_Adminhtml_Catalog_ProductController
 {

 public function editAction()
 {
    $productId  = (int) $this->getRequest()->getParam('id');
    $product = $this->_initProduct();
    $productName = $product->getData('name');
    Mage::register('previous_name',$productName);
    $productColor = $product->getAttributeText('color');
    $productBrand = $product->getAttributeText('computer_manufacturers');
    $ProductCompleteName = $productName.' '.$productBrand.' '.$productColor;

    $product->setData('name',$ProductCompleteName);
    if ($productId && !$product->getId()) {
        $this->_getSession()->addError(Mage::helper('catalog')->__('This product no longer exists.'));
        $this->_redirect('*/*/');
        return;
    }

    $this->_title($product->getName());

    Mage::dispatchEvent('catalog_product_edit_action', array('product' => $product));

    $_additionalLayoutPart = '';
    if ($product->getTypeId() == Mage_Catalog_Model_Product_Type::TYPE_CONFIGURABLE
        && !($product->getTypeInstance()->getUsedProductAttributeIds()))
    {
        $_additionalLayoutPart = '_new';
    }

    $this->loadLayout(array(
        'default',
        strtolower($this->getFullActionName()),
        'adminhtml_catalog_product_'.$product->getTypeId() . $_additionalLayoutPart
    ));

    $this->_setActiveMenu('catalog/products');

    if (!Mage::app()->isSingleStoreMode() && ($switchBlock = $this->getLayout()->getBlock('store_switcher'))) {
        $switchBlock->setDefaultStoreName($this->__('Default Values'))
            ->setWebsiteIds($product->getWebsiteIds())
            ->setSwitchUrl(
                $this->getUrl('*/*/*', array('_current'=>true, 'active_tab'=>null, 'tab' => null, 'store'=>null))
            );
    }

    $this->getLayout()->getBlock('head')->setCanLoadExtJs(true);

    $block = $this->getLayout()->getBlock('catalog.wysiwyg.js');
    if ($block) {
        $block->setStoreId($product->getStoreId());
    }

    $this->renderLayout();
}
/**
 * Save product action
 */
public function saveAction()
{
    $storeId        = $this->getRequest()->getParam('store');
    $redirectBack   = $this->getRequest()->getParam('back', false);
    $productId      = $this->getRequest()->getParam('id');
    $isEdit         = (int)($this->getRequest()->getParam('id') != null);

    $data = $this->getRequest()->getPost();
    $data['name'] = Mage::registry('previous_name');
    if ($data) {
        $this->_filterStockData($data['product']['stock_data']);

        $product = $this->_initProductSave();

        try {
            $product->save();
            $productId = $product->getId();

            /**
             * Do copying data to stores
             */
            if (isset($data['copy_to_stores'])) {
                foreach ($data['copy_to_stores'] as $storeTo=>$storeFrom) {
                    $newProduct = Mage::getModel('catalog/product')
                        ->setStoreId($storeFrom)
                        ->load($productId)
                        ->setStoreId($storeTo)
                        ->save();
                }
            }

            $this->_getSession()->addSuccess($this->__('The product has been saved.'));
        } catch (Mage_Core_Exception $e) {
            $this->_getSession()->addError($e->getMessage())
                ->setProductData($data);
            $redirectBack = true;
        } catch (Exception $e) {
            Mage::logException($e);
            $this->_getSession()->addError($e->getMessage());
            $redirectBack = true;
        }
    }

    if ($redirectBack) {
        $this->_redirect('*/*/edit', array(
            'id'    => $productId,
            '_current'=>true
        ));
    } elseif($this->getRequest()->getParam('popup')) {
        $this->_redirect('*/*/created', array(
            '_current'   => true,
            'id'         => $productId,
            'edit'       => $isEdit
        ));
    } else {
        $this->_redirect('*/*/', array('store'=>$storeId));
    }
}

}

Before Install this Module Create Backup of your site and Database. After Install this Module Flush All Cache.

It May be helpfull for you.

You could try something like this. I believe the below method is quite efficient.

$attrData = array(
    'attribute_code_here'=> 'Default Value Here',
);
$storeId = 0;
$productIds = Mage::getModel('catalog/product')->getCollection()->getAllIds();
Mage::getModel("catalog/product_action")->updateAttributes(
    $productIds, 
    $attrData, 
    $storeId
);