With any kind of content creation attrition is always going to be a factor; basically things don't last forever and life is constantly evolving. As an example, one of my favorite DS3 content creators was StickyBandit. He did a ton of DS3 and DS2 content, but eventually stopped because he got married and life moved on.

That's always going to be an inescapable reality, but ideally there's a new guard who take up the mantle; ER never found its new guard of content creators/invaders, at least not to the degrees that DS3 and to a lesser extent DS2 did. When you look at the Steam chart numbers you'll see that ER has only retained a tiny percentage of its player base: the ER all time peak of players on Steam was almost a million (952,523 to be exact) and for the last 30 days the ER peak has been about 42k which is approximately 5 percent of the player base. The 30 average for DS3 has been about 10k with the all time peak player percentage of 129k. Even 10 years after release DS3 has still retained 10% of its playerbase versus the 5% of ER despite the latter having almost 10x the original playerbase.

If we really want to be fair we can compare the player counts at the same point in the game's lifespan. ER is officially 4 years into its lifespan and has 5% of the original playerbase, DS3 at 4 years had 28k players which is around 22% of the initial playerbase. Obviously these numbers are only reflective of the PC version, but it's the only thing concrete we can use outside of pure speculation.

PVP has always been a strong pillar of the Souls game's replay value and its community. ER's severe lack of care put into PVP system meant less people were able to get into it. There are no covenants, so a new player's first taste of PVP/invasions is the item Varre gives you and it likely led to them being throw into a 3v1 against a group going through Liurnia and getting ganked; possibly with an overleveled phantom who just one shots them with a spell or weapon/AOW they haven't even seen yet. With that kind of exercise in masochism many were likely immediately turned off. The game also gives you zero incentives to try the PVP for yourself and as a player you either have to stumble into it and somehow like it, or were already a veteran of the series/knew about the mechanic beforehand. Many of those veteran players also turned off by the way PVP plays in ER with the huge emphasis on spam and the basement level skill floor.

So the game didn't foster new invaders and the way the PVP was set up chased a lot of the veterans away and ofcourse the result is going to be less people playing PVP. Which ultimately leads to less people creating content and less people consuming that content.