To sum up the problem with coalitions in the late-game very concisely, it's that the dominant expansionist strategy (the only thing you need to concern yourself with to expand optimally) is always to manage AE. Whatever you do, it has to revolve around minimizing coalitions. The player's ability to make strategic decisions is removed as a factor in growth and replaced entirely by their ability to develop and manage their AE spreadsheet. There is no situation where "do something that helps avoid coalitions" is a strategically inferior choice to "do anything else" after you become the dominant Great Power, hence there is no longer any strategy.
The irony of it all is that this is about the only thing everyone can agree on, and yet that's where most discussions get quagmired: there should be still be strategy in the late game. If we break down the discussion into two broad sides of "people who want coalitions relaxed" and "people who don't," then the former group holds their position because the current mechanics render "manage AE" as the only strategy, while the latter holds their position because relaxing AE, ceteris paribus, would render "expand as much as possible" the only strategy. That's why we shouldn't break it down into two groups, because everyone really wants the same thing, only it's not a matter of going left or right along some hypothetical coalition continuum--it's about coming up with an actual, elegant solution.
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