I can say that's not entirely true for some books. I have several books over 100K on Wattpad. A couple did continue to do well after hitting the 100k threshold, others died. Especially after they changed to the tag ranking system. That basically killed my reads over there. Before the tag system I was fairly consistently ranking in the smaller genres and getting a good amount of steady reads. Nothing amazingly spectacular, but steady. Then they switched to that system and everything went mostly poof. I probably have no right to really complain since I still had a small but loyal fanbase, but it really was night and day between the ranking systems for me.
The trick I think is in the engagement. I've never been the writer that gets a ton of comments, in fact my readers are typically silent readers but I did notice a difference in the books that had decent engagement vs the ones that had nothing. The ones that had nothing were the ones that died a lot faster upon completion regardless of them making it to 100K reads. I had one that hit 100K reads 2 years ago and it was only at 106k with no comments when I removed the chapters from Wattpad earlier this year while I try to decide if I'm going to stay there or leave completely.
It's hard to decide to leave there because I do have a fan base. They're just not a fan base who will follow me off Wattpad LOL so with a series still pending a third book, I do kinda want to hang on so I don't leave those readers hanging if/when I finally write it.
I don't see why it's a bad thing. The 2 books I have on here are both on Wattpad. Granted they're doing much better on Wattpad, but I'm also more established there and haven't really done what I need to do to get a steady fan base over here. But there's really nothing wrong with cross promoting. Unless like Domi said you wanted to enter the Watty's, but since it appears you don't then go wild!