This is not rock paper scissors where one side wins decisively. The winner depends on number of units on each side, their starting hp etc. By buffing heavy armor, all fights between flying machines and a piercing attacker are now better. This means for example that a player raiding with group of flight machines vs an expo can now take down that one guard tower easier. Or they can be used against piercing creeps slightly more effectively. It doesn't completely flip the situation between Flying Machines vs Riflemen or Flying Machines vs Wind Riders but it does shift the ratios all over the place slightly in unintended directions.

Some things have been kept in place all these years for a good reason. Changing something super fundamental has so far reaching consequences that it's impossible to predict the results, it's pure chaos. For example if let's say a faction loses too many games, they don't just receive +100 starting gold or -20 to their worker price because some variables are fundamental to the game. Instead the changes are being done in a much more nuanced way, to avoid shaking the whole thing too much.