First thing to remember is that Great Buildings are NOT supposed to be easy to get. If they were, there would be an option to buy them with coins or supplies.
The second thing to remember is that the only this game exists and is free to play is that it is a revenue generator for a business. The fact that people feel the need to buy diamonds in order to get the BP's they want is not an argument likely to effect change in a system which needs players to buy diamonds in order to be financially feasible.
You can get more than enough diamonds through regular game play to buy the occasional BP. Granted, there are other things you might want to buy with diamonds, so you'll go buy more. Shouldn't be a problem for people who spend hundreds or thousands of dollars on gaming consoles and games that don't amuse them half as much as Forge of Empires does!
The game isn't designed to be "won" or completed overnight. You need patience...in my case I've been able to complete BP sets for every Great Building available (except the 2 PME ones which I'm still working on) without buying a single BP.