I totally agree. It will be hot. But it's also hot today. And so you don't hear a lot of people saying "the sun will be hot" instead of "the sun is hot" because the former would be a meaningless use of the future tense.
But Christ doesn't say meaningless things. What he says is the very essence of meaning. So if he used the future tense he used it for a reason. If you have an idea for a different reason than what other Christians think, I'm all ears. But if you think there was no reason at all I would urge you to reconsider your unique opinion.
He was talking in human terms about something that has existed in the spirit realm. Catholics like to use that concept with the Eucharist but not with anything else. What's up with that?