It's ridiculous, and I am old enough and educated enough to do this whatever way I damn well please. Even after years of use, people still have to do just a touch of adjustment every time, and it is unnecessary. The whole thing is a pretentious convention - a trick, really - that has been around for so long that we will never be rid of it. But I'm just dropping it. When I mean that something is in the 1600s I'm going to write and say 1600s, not 17th C. The years of the 17th C all begin with 16, after all. Same with 3rd Millennium BC and the like. The whole thousand years each begins with a "2," after all. If I need to refer to the whole thousand I'll say 2000-3000BC or even just 2000+.
And frankly, even I seldom need to refer to the 5th Millennium BC in any fashion. Hardly anyone does.
But, but...what about the First Century, the plaintive cry will go up. Things have to be consistent, things have to match, things have to fit into a nice tidy order that goes all the way to the end. Well, no they don't really. If it makes you nervous that there's a hair out of place there I sympathise (I straighten other people's pictures), but really, it's a molehill. The number of times that we are depending on a full list where we have to put everything in a row is vanishingly small. Usually, when we are talking about a time period, it's pretty limited. Writing 1400-1700 is clearer than 15th-18th C. If I'm making that list, starting it
is going to be entirely clear. In sentence form, I probably will use 1st C, or First Century, and I will likely use 20th C and 21st C because we are so used to those that we don't need to mentally adjust. Though if I wanted to say a word was used more commonly in the early 1900s, that would actually be marginally clearer than "early 20th C." It types more easily as well - the latter has the same number of characters, but plus a space, switching between numbers and letters, and also switching to a capital letter. What's simpler about that?
In proper names, such as CS Lewis's Poetry and Prose in the Sixteenth Century (yes, he was the scholar chosen for that and it is still assigned 70 years later) I will of course not change the name. Though in discussing it I might repeatedly use phrases like "mid-1500s." Nor am I remotely interested in correcting other people, who might be more immersed in the convention or uninterested in putting any energy into a change. But I don't have to say 4th C and I ain't gonna.