Although this is not a WordPress specific question:
There is no need to tell it to get on with rendering the rest of the page. The else portion should contain what code you want to include if there is not a featured image. If you just don’t want to render that portion of the page then there is no need for the else.
Hopefully this makes some sense:
<header></header>
<if statement>
parallax div
</if statement>
<content.. etc>
Any code outside of the if statement will run whether the if statement is true or false.
Also, you do not need to echo all of the HTML code if you close the php tag prior to outputting your information.
<?php if ( has_post_thumbnail() ) { ?>
Plain HTML code here.
<?php } else { ?>
What happens if there is no post thumbnail.
<?php } ?>