I haven't been RPing as long as others in this thread (less than 5 years), so take my thoughts with a grain of salt.

I personally think Discord RPing is taking some of the userbase from Forum RPs. I've met a fair amount of Discord RPers who used to do forum but moved to Discord. I don't think it's "killing" forum RP or anything like that, but that's definitely one possible cause for some of the users leaving.

I also think that, in general, forum formats are losing users due to being forums. I think it's happening to almost every single forum (whether it's an RP forum, smashboards, serenesforest, whatever). It might just be due to how the internet is moving towards a more "commercial" service, where popular sites are owned and controlled by companies. Most forums are independent or user-run, and I think it's harder for them to keep up with the natural decline in any given userbase due to having less backing from a financial and commercial standpoint.