The problem I have with the type of solution you're suggesting is it protects players from the consequences of their choices and doesn't address the root problem. While it may appear the root problem of a city being plundered are the players who plunder it, I believe, the root cause is the player whose city is being plundered. Simply buying a boost or applying a potion to thwart the plunderers misses the point entirely. The solution to plundering is to address the root cause and that lies with the player.
To me, this is why the tavern boost is purposely expensive. Once you run out of silver you're once again confronted with the issue of a poorly designed and poorly run city built by players who lack knowledge of, or choose to ignore, fundamental principles of the game. Plundering is the pain the stimulates change, without it, there is none.
No one has forgotten about the lesser ranked players, we were all a lesser ranked players once. We've all had to deal with being plundered repeatedly and we all had to figure out how to build a city that doesn't repeatedly get plundered. This is one of the many challenges presented by the game that players need to overcome.
Which, to me, is why Inno put it on the Do Not Suggest List. They're basically saying, "This is how we designed the game, don't ask us to change the game, learn to play the game we designed."
Because that's the game.