Nah. It's more common than you think, you just need to know where to look - a LOT of forum RP happens on forums that aren't actually dedicated to RP, for example. I started 7 years ago in the Flight Rising forums, and my boyfriend to this day RPs exclusively on Paradox Interactive forums, to name just two. 

Also, don't take a perceived slowdown of this site as evidence of a decline. Firstly, the site goes through slow and busy seasons, and it's also entirely possible you're just not looking in all the right places. Secondly, I'd wager that half or more of the RP that happens here happens in PMs, not in threads, so there's that additional truckload of RP happening where you just can't see it. There's also a ton of discord and google docs rp - not technically "forum" RP, but written RP nonetheless. (Yes I know OP wasn't talking about specific sites but other people have, sue me)

As well, like someone else said, age is a factor too. I started as a young teenager and things felt way more active back then, but then again I have no way of knowing if they actually [i]were[/i] more active or if it just [i]felt[/i] more active. For all I know, my experience in the format and managing RPs and posting and all that might just make things seem slower because I've gotten faster at writing or otherwise more efficient with my writing time.

Plus, people tend to gravitate toward their own age group (generally speaking, obviously) so it's also possible things feel slower because your age group is now older with more shit outside of the internet to be doing. 

I don't think forum RP is really going to go away, since at least for me, there's nothing else that scratches the same itch. Writing solo loses the fun social aspect of RP, and IM RP or D&D or in-game RPing or whatever don't give you the same narrative freedom. That might only be relevant to people like me for whom the actual [i]writing[/i] part (not just the role playing) is the fun part, but I doubt I'm alone there.