Ok. Ist working, but:

f i try to build a static footer with just static HTML, its not possible to replace the "default content".

Trying to remove 

<div class="oe_structure" id="footer"></div>

<template id="footer_custom" inherit_id="website.layout" name="Footer">    
  <xpath expr="//div[@id='footer']" position="replace">
        <div>static content</div>
   </xpath>
</template>

causes an error.

And just editing id=footer will be ignored

<template id="footer_custom" inherit_id="website.layout" name="Footer">            
    <xpath expr="//div[@id='footer']" position="replace">                
        <div id="footer">
            <div>new content</div>
        </div>            
    </xpath>        
</template>

it looks like replacing contents inside of if=footer is not possible.

<template id="footer_custom" inherit_id="website.layout" name="Footer">
    <xpath expr="//div[@id='footer']" position="replace">
        <div class="oe_structure" id="footer">
            <section>
                <div class="container">
                    <div class="row">
                        <!-- content -->
                    </div>
                </div>
            </section>
        </div>
    </xpath>
</template>

I think this would be the way to replace the footer.