The answer is in the writing of the easement. If an easement is properly drafted, it will stand the test. There are many cases of just such an attempt where the courts have upheld the powers of well-drafted easements. The land's value is reduced - since it can no longer be developed. This is not to say that it will not appreciate as the general overall real-estate market appreciates, but the market for restricted property is different.
Land is a limited commodity.
As time marches on and open land becomes scarcer, a strong argument can be made for a consequent increase in value. The land can be sold on the open market but when it is sold, it is sold with this permanent restriction. It can be sold many times over but the restriction remains. Obviously, it is a different type of market - not presently a speculative one, and the values are lower but surprisingly, the values are not vastly lower. The land may be fertile farmland or have some other potential value in which case it becomes a rather unique commodity.