I fully agree that the next SH has to be someone we have known for years.
There's
apparently 4-5 years left of OP. The journey is almost finished. By the end of Wano, the straw hats would be recognised as a fully fledged yonko level threat. That idea has already been teased by Morgans after WCI, and when they defeat Kaido it will become a reality. They will officially be at the top. And the battle will be one between equals at the summit, rather than those on their way up.
Anyone who joins the crew when they're already at the top would simply be piggybacking on their glory and would not feel like a true member of the crew to the readers, or in the OP world.
Even for Jinbe, who only just joined recently, he'd been in the process of doing so since 2012. It wasn't like pre timeskip where a character can just join the crew in the arc they're introduced. They need to be with the crew for an extended period of time for them to really feel like a part of the journey.
And that's exactly what Oda seems to be doing with Carrot. Introducing her in Zou, having her randomly travel to WCI, for literally no reason other than "FUN", having her then travel to Wano, and now having her as a key figure alongside the straw hats in the main operation.
It would feel out of place and arguably undeserved to just have Carrot join in Zou, having missed so much of the journey so that's why I believe he brought her along for the ride in WCI, one of the longest arcs in the series so we can feel accustomed to her with the crew, so she too can play a big role in their adventure (having saved their lives even) so when she officially joins, things fit as she's already been in those massive landmark moments with the crew.
So that her being in moments like this isn't strange or forced, but simply natural as a straw hat pirate herself.