No, it is an apt comparison. You're basically saying if character X wasn't added in, we'd get character Y, despite the fact that it ignores what goes into adding those different characters and the process behind them. That's exactly what someone who'd say
are a waste of a slot, as they ignore the decision and planning process, the development of their moveset, time allotment, ease of development, etc. Additionally, while we get characters every two weeks, remember that Smash is cut off as soon as the base game and DLC is done, so you have a small newcomer roster of say 10 to 15 + about 5 and that's it. Heroes is ongoing and as long as it's active characters will always have a chance to be added in the future (especially since influences like CYL are taken more into account), so the number of 'slots' are as many as the future allows.
I won't pretend I don't understand your misgivings, but there is little difference here. You're arguing from the perspective that a character slot is better used on someone else, and no matter of trying to explain it away will change that.