I feel like no one properly addressed this point so I'm going to give my thoughts on it
I don't think the exclusion of a large part of the player base is arbitrary at all, it encourages players who are not at max rep or don't have the gear for it to continue playing the game and grind for it. If you're arguing that you can't get things like umbral forma or exclusive mods without reaching high nightwave levels, then that's moving away from the tridolon and profit taker bounty specifically and more to the topic of umbral forma forcing players to do nightwave.
In terms of the no real gain to anyone else part, I think that things like encouraging players to do profit taker and tridolons is healthy for the game as it gives variety, tridolons and profit taker are different experiences to the standard missions in warframe. Players are constantly asking DE for more content and ngihtwave is a way to get players doing existing content that they may not have reasons to do otherwise but still get satisfying rewards nonetheless, if the player doesn't enjoy said content they don't need to complete every challenge to get all of the nightwave rewards.
To sum up I think the inclusion of profit taker bounty and tridolon cap is good game design as it encourages players who aren't at the level to continue playing the game and get to a level where the fights are accessible and doable, Its beneficial to veterans who can do these fights as it gives more variety to gameplay.