Would you rather have 50 useless units or 4 that kill those 50?
If I pull a useless 5* unit, then I might use them for SI, but those are pretty rare cases because most 5* units aren't useless. Usually even off-meta units are still pretty good at something at 5*, and it may be what takes me into a higher arena tier that week if it happens to be a bonus unit. If I sacrifice my only 5* unit of a certain type and it happens to be a bonus character next season, then that would be pretty bad.
I would rather have a roster of 12 decent units that have a chance of being a bonus character in the next arena or tempest than give up a bunch of them to make one unit super strong. My roster isn't that deep to begin with, and I'd rather spend my feathers on promotions to round out my teams. This is especially important since with chain battles and the like, the game seems to want you to have a deeper and more varied roster instead of four super powerful units that carry you through everything. It would be boring to use the same team every fight anyway.
Yes, some units can be super strong with lots of 5* investments, but I don't have the resources to spend on that at the moment. Maybe after another 5* promotion or two.