You're glossing over some of the complexities of this rather interesting situation.
In particular, the guy who wanted to be able to make it a compile-time option had a program that would do basically nothing without readline, and you had to enable it for the program to work. A similar issue is with two-part programs; one part non-GPL, one part GPL, which are left up to the user to link himself.
What matters is not what RMS says (nobody cares about RMS), but what the lawyers say. Apparently, the law is extremely harsh on this sort of "hack", which may seem fine to a programmer, but to a judge will be seen as an obvious attempt to work around the terms of the license and will be greatly punished.
In particular, the guy who wanted to be able to make it a compile-time option had a program that would do basically nothing without readline
That's not true. If you read the story you'll see that the guy had a working program for years before he used readline. The use of readline only improved an already working program. Not to mention, the program in question was a language implementation and readline had nothing to do with it. It was a user interface perk.
At one point he said he'd distribute a working program without readline or readline support even enabled by default and leave it up to the user to configure it. It was only going to have an interface to readline without including any readline code. He was told he couldn't even do that.
In the end he chose to GPL the entire language implementation. To him, it probably didn't make a difference what the license was and it was easier than replacing readline. Personally, I would have replaced it with a BSD-licensed alternative just out of spite given the way the whole thing was approached. The line about how a judge wouldn't be convinced the program didn't violate the GPL read like a thinly veiled threat to me.
At one point he said he'd distribute a working program without readline or readline support even enabled by default and leave it up to the user to configure it. It was only going to have an interface to readline without including any readline code. He was told he couldn't even do that.
Then RMS was wrong. The SFLC says you're quite allowed to do that, a classic example being ffmpeg, which can be LGPL, GPL, or even non-redistributable depending on what external libraries you link it to on compile-time.
People have to learn that RMS is not God and that just because he says something about the GPL doesn't mean that the lawyers agree.
What if there was a proof-of-concept of a forthcoming replacement for the GPL'd part, and it happened to be vaporware? I'm not too familiar with all of this.
The farther you get into the gray area, the less of a clear answer you can get, because it becomes more than anything else an issue of which way the judge rules. This is why generally it's a good idea to err on the side of caution with any type of software license.
You're glossing over some of the complexities of this rather interesting situation.
In particular, the guy who wanted to be able to make it a compile-time option had a program that would do basically nothing without readline, and you had to enable it for the program to work. A similar issue is with two-part programs; one part non-GPL, one part GPL, which are left up to the user to link himself.
What matters is not what RMS says (nobody cares about RMS), but what the lawyers say. Apparently, the law is extremely harsh on this sort of "hack", which may seem fine to a programmer, but to a judge will be seen as an obvious attempt to work around the terms of the license and will be greatly punished.