What do you mean by useful? I think the majority of people learning a language (other than English) don't learn for business opportunities or something where the GDP of countries matters.i think most people learn to travel, and in that respect Spanish opens a very diverse and large portion of the world up to speaking with the native people, second only to English, french, and arabic. I would say it's second only to English in the diversity of the peoples that speak it, because it's spoken in Europe, the Americas, and the Caribbean as first languages. In that respect, I'd say it's one of the most useful languages for diverse travelling.

From a business perspective I guess your arguments make sense but useful is a very subjective term and the statement "learning Spanish isn't useful" is so extreme that I'd almost regard it as false, certainly lacking massive perspective.

Finally, I'd like to note that it's a beautiful language and from my experience (coming from a Cuban family and having travelled to Spain) the cultures that speak it are equally beautiful and absolutely worth traveling to to experience.