"Because it could be bad" can apply to everything.
I think a familial bond can be used (and of course, would be used for the majority of the Toa i.e. the ones not in a relationship with other Toa in the group) quite well, but I don't see how that would be "better". It's simply a different type of relationship that people have with each other.
Also, "feels more Bionicle" is something that's pretty subjective to a lot of our fans, and there's things that some people think that "feel Bionicle" that we've already written off, like it being constraction, or genders being locked to elements, or a five to one gender ratio. We wrote those off because we figured out the core of Bionicle (worldbuilding/aesthetic) and are throwing out the arbitrary rules and restrictions that previous generations had. "No romance between the Toa" falls into that category.
Here's a list of things that can "mess up" character dynamics in a story:
- People hating each other (initial Tahu and Kopaka)
- People liking each other (Kopaka and Pohatu)
- People maturing (Tahu)
- People growing in power (Takua/Takanuva)
- People dying (Matoro)
- People turning evil (Lewa, Tahu, Vakama)
My point is, character dynamics are supposed to change and grow. That's the whole point of character development. As they change, the dynamic changes. I hate it when people use the term "mess up" as that denotes a negative change, and implies that every romance will end in a negative change in a group dynamic. Every relationship is an avenue for character growth, and romance isn't any different from that.