Daisy is a godawful Echo and not a good standard for making anyone an Echo. Like, even Richter is remotely better, if barely. Dark Samus trumps both with an actual decent gameplay elemental change.
Besides, even with Rock Pikmin, those would already require actual differences due to having a different design from Purple Pikmin. Not "barely different hurtbox" like Daisy has. She's 99% the same, which is why she's a poor example of anything. I mean, she's the only Echo that people think should be cut because of the lack of differences being meaningful. Richter is given slightly more of a pass since they're more plausible in combat too, same as Dark Samus like I said above. If only just barely.
Something else to remember is part of the reason Alph is still a costume; he doesn't actually have any unique skills that Olimar doesn't have. In addition, the Pikmin are one-half of the full character. So unless they think of clear differences or new Pikmin come along, there's zero incentive for a separation(the actual character is basically Pikmin & Captain. Don't let the poor translation of their name fool you into thinking it was ever otherwise). The only bad part is the personality, and Olimar suffers equally. Both have far more personality than that and are inaccurate in that regard. Once you give Olimar personality, then you already have a decent reason to separate Alph.
Besides, at the time, there were no new Pikmin games coming out, so even without new Pikmin in general to add to make the moveset actually change a bit more properly, at least then you could use some unused Pikmin for that but the overall franchise would be more active enough to justify an Echo. I mean, Rock Pikmin aside, Bulbmin are also unused. It's not hard to separate them while being faithful to their personalities(that means actual better taunts). Next game is plausible, assuming the new Captain isn't used instead, which would be awkward for Alph fans, won't lie. Though I can hope at least the new Captain has a similar alternate costume, at least?
Daisy isn't in the same situation as she wasn't an actual costume for Peach but a reference at best. Even the most faithful one in Melee was closer to a cosplay than an actual alternate character design. Even then, with how terrible she was handled(seriously, you can't use one single Custom Move?), she doesn't set a good bar for creating an Echo that actually can be remotely faithful. She doesn't even have the excuse Simon and Richter had where they were intentionally based around each other's abilities. She's just there for the fans, but lacking a lot. The Toads still make little sense, and while you'd have retool her aerials and playstyle majorly for the floating to be gone(which is fair why they don't change this), she could've even had slightly different Vegetables. Like, say, Carrots from Super Mario Land. At least her Final Smash makes sense as a pure aesthetic change. Eating daisies are healthy, so it directly has a bit of a genius bonus to it.
But long story short, Daisy being a crappy Echo doesn't really matter because Pikmin & Alph have a different situation going for them beyond simply being somewhat easy to separate in theory.