Of couse Sirius wasn't replaceable. In fact, Remus rather thought that he would be quite lost if he woke up one day to find that Sirius was gone. The mere thought of it left Remus' chest feeling a little constricted. But he wouldn't admit that out loud. There was no need to feed Sirius' ego. Remus would hear no end of it. Besides, he was pretty sure Sirius knew it.
Remus followed a respectful distance behind Sirius, now unable to hide his grin. He kept a few feet between them, just in case Sirius decided to resort to underhanded means to make Remus admit it. Like tickling. When Sirius suggested that he would replace Remus, Remus merely grinned impishly at his friend, saying, "You won't. If you do, you won't have History of Magic notes to copy. You also won't be able to find your socks in the morning. And nobody will help you stall your jilted girlfriends when you decide to break things off. I'm indispensable," Remus concluded smugly.