In the English language, here is no such thing as a future tense. There are only two tense: the present tense and the past tense. We are capable of discussing future time but we use auxiliaries to do this; we do not alter the verb form to show "future tense."
E.g. I will finish this book later. "Will" is the auxiliary that is used to refer to the future, but it is present in form.

The past tense does, however, have a verb form designated to it (i.e. the -ed form) and qualifies as a tense; so does the present tense with the base form of the verb and the -s form of the verb.

Since there is not a verb form designated to the "future tense" then we say that there is no "future tense."

Sources

Lecture from Professor Eble (UNC Chapel Hill professor of grammar)
Kolln, Martha. Understanding English Grammar. 8th ed.

I think you're over-thinking this (or at least your source is). Also, I'm not sure it's a helpful way to look at tenses. I would say the future tense is formed by using an auxiliary verb as opposed to the future tense doesn't exist.

There are, of course, lots of other tenses in English other than past, present, and future. Let's take the past progressive, the present progressive, and the future progressive. You will doubtless know that they are formed I was going, I am going, and I will be going. I would call them examples of the past tense, present tense, and future tense respectively.

All that aside, even if the idea that a tense has to have a verb form designated to it to be a tense is right (which I cannot agree with), then check this out:

The next train arrives at 4 o'clock.

How's that for a verb form specifically used for the future tense?

Saying the future tense doesn't exist is unhelpful IMHO. Also, I'm pretty sure I could find you a dozen professors who disagree with Professor Eble, so we need to be careful not to succumb to Authority Bias.

I can see how this idea might keep a few academics entertained for a minute or two while they're having a cup a tea, but discussing it beyond that would be damaging to learning.

Anyway, I've nearly finished my cup of tea now, so let's have no more chatter about there only being two tenses.

Yeah you are right. There is a Future tense even that is the topic am teaching my students this week. Future tense same as Future Progressive tense. Just that some Scholars believe there is nothing like future tense. Anyway no need to argue because you have said it all.