I set off quite the discussion didn't I with the "heavy cavalry" comment

To be clear I have no idea if there will be a light/heavy cavalry distinction in Civ7.
I have no argument with you in terms of historicity. I greatly appreciate your extensive knowledge on the subject and contextual contributions - I find them quite educational. What I am saying though is that the popular conception of tanks is "hit fast hit hard", pop blitzkrieg if you will.
Since Civ has abstracted military units into distinct "lines" it doesn't make sense from a gameplay perspective to have tanks start in the siege line then move to the cavalry line. It would be confusing to most players and mess with upgrade directions. I could be wrong, but I'd be very surprised if we get an accurate depiction of early tanks as they don't fit nicely in the abstracted lines already defined.
I will also say that I hope the modern age doesn't present us with just 20-30 turns to use units before they become obsolete - I suspect further abstraction or blobbing of the rapid technological innovations in the 20th century to avoid the frustration of not getting to use your units before needing to upgrade them. At least, I hope they address this issue, as it was just another issue plaguing the late game. I honestly wouldn't mind the modern age ending in the 1950s as we would avoid having handle an absurd amount of military innovation in a really compressed time.