But this all also applies to Iceborne, yet I don't remember much real talk about a tepid launch or needing a rebound for it then?
There was definitely talk of how Capcom could've been more timely with Iceborne's rollout, back in the day. There was also talk of how the game was selling to a smaller percentage of MHW buyers than people expected, but at the time it was chalked up to MHW itself having attracted so many new players that a lot of them had simply had their fill and moved on.
This assumes two things that I don't think the data currently supports
1. That other regions buy expansions at the same rate as Japan
2. That the World/Rise audiences are as likely to buy expansions as the Gen 4-and-older audiences
I think the key question there would be "why." But beyond the product itself not being enough of a draw, I can't really think of any reason for that. In the case of Japan specifically, it would mean that MH has declined enough to where expansions aren't as big a deal anymore. As for the rest of the world...I don't know. This game launched globally on two extremely successful platforms. There's an extremely large potential market for it.