Replace Add to cart button with a contact form if product is out of stock in WooCommerce

With help from the community, I have successfully gotten the behavior I want with variable products, but I can't seem to duplicate it on simple products.

The code used is inserted into the functions.php file of my theme:

function woocommerce_get_custom_availability( $data, $product ) { 
    switch( $product->stock_status ) { 
        case 'readytoship': 
            $data = array( 'availability' => __( 'In Stock','woocommerce' ), 'class' => 'ready-to-ship' );
        break; 
        case 'outofstock': 
            $data = array( 'availability' => __( 'Call for Quote', 'woocommerce' ), 'class' => 'out-of-stock' ); 
        break; 
        case 'onbackorder': 
            $data = array( 'availability' => __( 'On Backorder', 'woocommerce' ), 'class' => 'onbackorder' ); 
        break; 
    } 
    return $data;
}

add_action('woocommerce_get_availability', 'woocommerce_get_custom_availability', 10, 2);

add_filter( 'woocommerce_available_variation', 'variation_out_of_stock_show_form', 10, 3 );
function variation_out_of_stock_show_form( $data, $product, $variation ) {
    if( ! $data['is_in_stock'] )
    {
        $data['availability_html'] = '<div class="contact-outofstock">';
        $data['availability_html'] .= 'Please contact us for a quote and availability.';
        $data['availability_html'] .= do_shortcode('[gravityform id="10" title="false" description="false" ajax="true"]'); // Replace with your own contact form shortcode
        $data['availability_html'] .= '</div>';
    }
    return $data;
}

add_action( 'woocommerce_single_product_summary', 'hide_add_to_cart_button_variable_product', 1, 0 );
function hide_add_to_cart_button_variable_product() {

    // Removing add to cart button and quantities only
    remove_action( 'woocommerce_single_variation', 'woocommerce_single_variation_add_to_cart_button', 20 );
}

But all variations on the above that I have tried for a simple product, both in the functions.php file and /woocommerce/templates/single-product/add-to-cart/simple.php just provide the default behavior (or produce a critical error).

Any suggestions?

There are some mistakes, missing things and also unnecessary things, to make it work for simple and variable products.

Replace all your current related code with the following:

add_action( 'woocommerce_get_availability', 'filter_wc_product_availability', 20, 2 );
function filter_wc_product_availability( $data, $product ) { 
    switch( $product->get_stock_status() ) { 
        case 'readytoship': 
            $data = array( 'availability' => __( 'In Stock','woocommerce' ), 'class' => 'ready-to-ship' );
        break; 
        case 'outofstock': 
            $data = array( 'availability' => __( 'Call for Quote', 'woocommerce' ), 'class' => 'out-of-stock' ); 
        break; 
        case 'onbackorder': 
            $data = array( 'availability' => __( 'On Backorder', 'woocommerce' ), 'class' => 'onbackorder' ); 
        break; 
    } 
    return $data;
}

add_filter( 'woocommerce_get_stock_html', 'filter_wc_get_stock_html', 10, 2 );
function filter_wc_get_stock_html( $html, $product ) {
    if (  $product->get_stock_status() === 'outofstock' ) {
        $html .= sprintf('<div class="contact-outofstock">%s</div>', 
        esc_html__('Please contact us for a quote and availability.') .
        do_shortcode('[gravityform id="10" title="false" description="false" ajax="true"]') );
        $html .= '<style>.woocommerce-variation-add-to-cart-disabled {display:none;}</style>';
    }
    return $html;
}

It should better work.