Is planting trees on good land crazy or immoral ?

Planting trees will drastically reduce the value of your good land (I mean in terms of what someone would be willing to pay you for it)

Advice would be to rent it to someone else instead. Long term lease of 5 years is tax free income. Require the money up front at the start of each year.

Certainly not immoral. I would say the opposite given the dearth of tree cover in this place. The west of Ireland looks like a desert because of overgrazing. I may move rural soon and hope to have a couple of acres to spare. If so I'll be planting a mix of trees types which I appreciate not just for their beauty but also for their crop such as hazel and apple etc. If a damned sheep wanders on to it he'll be a gonner.

“A society grows great when old men plant trees in whose shade they know they shall never sit” – an ancient Greek proverb?

Good land for what OP? Can you grow a crop of barley, wheat or oats on it? Would it be good for potatoes or carrots? Perhaps its only "good" for grass if so its not that good.

Do you have to make money from the land? If that isn't important do wtf you like with it you could even cover it in solar panel or put up a wind turbine and still graze it.