Yes, in real life, you could do so; IRL, you could also pay workers overtime no matter how "Wonder"ful the building in question is. And a brand new Colosseum or "luxury" spending wouldn't calm down protestors against war and/or draft.
Civ is a game, not a representation of the world; part of what's fun about it is the fact that it draws everything from real history, but it is still a game, and we should work within its rules. While gathering information about what would happen if you did X and then reloading isn't breaking any rock-solid rules, it does remove a lot of the risk - and, subsequently, a lot of the decision making - from the game.
Besides, if we played ahead on all diplomatic deals, the FAM would have a pretty boring job.