What makes you so sure of that? They've used the female trainer of the generation (or Lillie) to fill the female companion role ever since Hoenn, not a Water gym leader like Misty. Not to mention the relatively nonstandard format of the Sun/Moon season. Actually, due to how corporate the whole Galar league is, I don't think any gym leader has the luxury of just hiring a replacement while they go travelling: their sponsors probably want them to be available at a moment's notice.
Not saying it's impossible Nessa will be more populaar at the end of the day, but I don't see any obvious signs of her being more significant at the moment.
Back to Bea, I realize there's another fairly recent Nintendo female who wields a steely gaze and martial arts...
She won a massive popularity contest spannning several weeks in her game, by the way.