I'm not sure I fully agree.
The more different viable leaders there are, the more usable subs there are that work with those leaders. Goemon's subs are different from an otherwise ordinary mono-red composition, where Uriel/Noel, Ares, and Freyr/Tyrannos are swapped around as needed for the multipliers desired. Isis/Horus/Ra demand a rainbow composition, and Valkyrie healer teams have their own lists.
There are a lot of monsters that are included on several lists, like Echidna, but for the most part, I don't feel like the team building diversity is a significantly limiting factor. The upcoming limited-team cost dungeons restrict leaders and subs, and force new monsters to be used, though rather artificially.
I started playing with Valk + healers, switched to monofire Freyr, added an ADK team, used some Siren/Odin resists in the middle, and eventually started tackling Descendeds and added Goemon. Hera's not much of a leader, but Zeus used to be, and is still a great sub. Upcoming Athena and Zeus-Dios are also excellent leaders.
I feel like there's a fairly healthy list of great leaders, many of them from Descended dungeons. While I don't disagree that Lucifer is easymode, it's not a perfect team that can tackle everything.