Mana is constant because it decays at the same rate as your tokens generate it.

If you hold real estate and rent it, do you generate money from thin air? Is it risk free? No. And same is true for IOTA, the value of the token fluctuates in the same way real estate prices fluctuate. This is not a risk free investment. If you bet on it you bet that it's going to be adopted. Everything that gets adopted increases in value. It's up to you to do your research and understand how this works and why this works and come up with a decision that is suitable for your situation.