Footers do exist for reasons, not the least of which is for legal reasons. The requirement of displaying a footer will lead designers who use an infinite scroll into solving the footer requirement by making it float. Floating footers, by their very nature, will lead the users into believing that they have reached the end of the page. You can’t depend on users noticing “cut off” text above the footer (if it happens to occur). Neither can you depend on users noticing the size and position of the scroll bar, especially now that browser interface design is minimizing the scrollbar’s appearance. Floating footers run the risk of negating the proposed value of the infinite scroll. If users think they are at the end of the page, the content below the fold is effectively made invisible to the users.
In my experience, it is only advisable to use an infinite scroll if the context of the application is one in which it is expected by the users, like social media, collaboration applications, etc. In an of itself, volume of content does not warrant leveraging an infinite scroll when pagination makes it more usable. Even Google, who arguably presents a context in which content is virtually endless, does not present an infinite scroll on their interface.