I'd consider all fixed UI elements to be obtrusive. It's a question of whether the utility justifies taking up space on screen (semi) permanently. The functionality provided by such floating UI elements is rarely important enough to block an entire part of the screen and you also risk blocking page content.
I do not like the back-to-top button either, because going back to the top of a web page is a solved issue in pretty much all browsers on any operating system. It can, however, be argued to be useful for less tech savvy people and is at least usually hidden on a forum where you tend to read scrolling downwards.
A scroll-to-bottom button would, conversely, be visible most of the time. That would be fine if the utility was there, but I'm not sure what exactly this button is supposed to accomplish. Even on long threads it only takes a few seconds to scroll to the bottom of the screen.
I certainly agree there is room for UI improvement. Perhaps you could elaborate on how specifically such a button would improve your experience, how it would save you time when navigating the site, and how often you run into issues that could be solved by the addition of such a button.
Thanks for coming to my TED talk on why floating buttons are usually a waste of space.