They’ll likely carry over existing characters because:
They’ve been doing it since G1. There’s always been an Applejack. Rainbow Dash, Fluttershy, Pinkie Pie, and Rarity were in G3. The names are already trademarked. It saves them time, money, and possibility of error in coming up with new names that even can be trademarked, and trademarking them. (Look how they screwed up with Trixie and Coco just to point out a couple, and the fan backlash when they had to rename Lyra to Heartstrings. And how many they found out they couldn’t trademark without adding Dazzle to their names.) Plus it preserves the trademarks they already have going back in some cases to G1. Not just Applejack, but Minty, Bon Bon, and Surprise. And should’ve been Firefly, but they lost the trademark on her name through disuse. A mistake which they don’t want to repeat by not using anypony they already have. (Even if G5 is awful, they still want to own those trademarked names for G6.) Not to mention, hopefully, Spike.
It adds continuity and nostalgia value to the franchise. G5 Rainbow Dash may not be my
Rainbow Dash, but my grandniece likes her, I can relate. Just as happened with many G1-3 fans before. Plus since eventually you won’t be able to get new G4 any longer, some of the fans who aren’t instant adopters will still move on to G5 eventually.
It gives extra value and salesworthiness to existing toy stock/molds/decals/books/etc., until they run out/wear out.
Or until Hasbro can get a handle on just how much G5 merch they’ll be able to sell and ramp up new production to match demand. Remember how long it took to get stores full of reasonably on-model G4 merch? Just because G4 toys will be off-model to the then-current G5 characters doesn’t mean toy store buyers, parents, or even all children and adult fans will care, if they can get the characters the children want. Funko even has several lines of figurines whose specific appeal is that they have a unique look that’s utterly off-model to everything. And McD’s was still pushing a handful of G3 toys out as unused overstock to replace running out of the new ones as recently as last year.
It gives a hook to existing fans. Sure, there’s a possibility of backlash if we find out for instance that G5 Rainbow Dash always dresses in style again, and nobody wtcig5rd. Or G5 Applejack is an angry inner city gangbanger with a secret heart of gold, or some other character stereotype everyone hates. But assuming they don’t totally screw it up, aren’t you curious what the new G5 Mane 6 will be like? Would you be as curious what Pop Bottle, Marmalade Muffin, Grassy Meadow, Trotsalot, Eagle Wings, and Dragonfly Delights are going to be like?