There should be variety in how maps are completed. Not always bosses.

Some maps should be completed by killing a boss.
Some maps should be completed by killing a bunch of rares.
Some maps should be completed by some interaction objective.
Some 4th option.
Some 5th idea.
A 6th thing.

No version of "this is what completes a map" should be applied to every map.

Some maps should indeed just have a "corruption radiating enemy" in the form of a boss or several rares.

Other maps should have objects we need to cleanse of corruption with a quest item pickup, like how we mess with the Sacred Wells in Qimah Reservoir in the Interludes.

Some maps should just have a magic map completing button to push like how the tower maps are completed by just clicking the beacon.

There could be map nodes specifically dedicated to a given league mechanic, that are completed by doing all instances of that league mechanic.
-A special ritual map with 5/6/7 ritual alters needed to finish the map.
-A special expedition map that's like being on Plunder's Point again.
-We already have "Overrun by Abyss" maps, just making clearing all the abyss openings the objective.

There's various options for how a given map could be completed.

The main point here is just use multiple options.
Don't make every map completed the same way.

Welp, every single map being a boss map already feels equally as boringly repetitive and redundant as every map being a rares map did.

Variation is a spice that helps keep games fun, could really use some.