I haven't used Lisp but I've used another environment that was heavily image based (Frontier). The runtime ability to modify things sounds super neat but in my opinion it leads to disaster. It is not a weakness that we have a lot of tech built on a concrete specification of the initial state of the system and very careful control over where data and the modified versions of that data live. It is hard-won experience that it is a good way to build robust systems. "Reboot & pray" isn't just an accident, it's a legitimately good way to set up a system.

Some of you may object to the idea that we are super careful with data and modifications to it. I know where you are coming from... however, compared to an image-based system, the modern world is in fact super careful! To the extent that you think we are still not careful enough, that leads you even farther from wanting an image-based system.

If you conceive of a system as the full state space of everywhere it can go, the vast, vast, vast majority of systems have pathological places in it. It is very hard to get them all out. It is advantageous to have a button that says "restart this system from the initial state", and to push it often to make sure it continues to work, even if you're not thinking of it this way. By contrast, image-based systems have the tendency to either 1. get into a bad place in the state space and then the user has no way back out or 2. accidentally create a scenario in which there is no way to bring the system up from scratch anymore, which includes things like "giving the system to someone else to start their own work up".

Sure, it's a good idea to minimize the need to hit the reset button. But you don't really want to give it up entirely.

There are a variety of ways of improving the image-based system. Most, perhaps all of the practical ones, involve basically removing the image-based nature of it and moving closer to the systems we all work with today.

I don't object to any of this. "Restart this system from initial state" is quite a bit more common than people imagine in CL. I was hinting at it perhaps being even more common than image-based development, though don't quote me on that. I personally don't even really create Lisp images in dev (save for creating executables that I don't then further modify, which are technically images in SBCL).

Incremental runtime modification is great for development, but I very frequently restart (from sources) every so often, and certainly at least before deployment to verify everything is working as expected. Sometimes because it's completely necessary, sometimes for my own sanity.

For production, I think it's a very "neat" thing to be able to do and I've heard some great stories (no personal experience), but it's definitely a foot gun if not used very carefully.