Learn Russian: Спрос на этот экологичный тренд скоро взлетит. - Demand for this eco-friendly trend will take off soon.

The noun спрос (demand) specifically requires the preposition на plus the accusative case to show what is in demand. It is a fixed pairing you just have to memorize, similar to how English uses 'for'.

What is the difference between экологичный and экологический? Can they be swapped here?

They cannot be swapped. Экологичный describes something that is 'eco-friendly' or safe for the environment, which fits this trend perfectly. Экологический is strictly related to ecology as a science or sphere (for example, экологическая проблема - an ecological problem).

Is взлетит used metaphorically here just like 'take off' in English?

Yes! The perfective verb взлететь literally means to fly up or take off, with the prefix вз- indicating upward movement. Russian frequently uses it metaphorically to describe a sudden, sharp rise in things like demand, prices, or popularity.

Why doesn't the ending of этот экологичный тренд change after the preposition на?

It actually does change grammatically: it is in the accusative case because it follows the preposition на. However, because тренд is an inanimate masculine noun, its accusative form is identical to its nominative form, so the words look unchanged.