This is a really interesting question. I'm not certain that I have a good answer for the question, but I believe the opposite, where I think there's an eventual limit on the difficulty. Level difficulty is one of those things that's very subjective. When it comes to describing the general difficulty of a level, everyone's views will differ. I'll take United as an example. So far, only two people, Armani and me, have completed the pack in its entirety, where I recently finished it. He and I have differing views on levels that we thought were difficult. In particular, it seemed that Armani found Genocide 37 and Genocide 41 to be really hard, which I agree to an extent, but I didn't think either one were THAT hard. Sure, they still took me a while, but I was able to solve them somewhat fairly quick. In contrast, even though I figured out some things really quickly in Genocide 39, I struggled a lot with it and found it pretty difficult while he didn't. As these examples show, levels that some people found hard may be easy for others, or vice versa. However, if most people found a particular level hard, then it's generally seen as a hard level, even if some found it easy, or vice versa.
There are some people who have most of the solution to a level, but they are missing one very important key element that is preventing them from solving the level. Generally, these are levels requiring a certain trick and can either be difficult because the person doesn't know about it already, or they can be really easy because the person already knows the trick needed. A really good example is Tension 34 from United. Before when I didn't know the trick, it was a really hard level and was a roadblock for me for a few weeks. However, once I discovered it, then it's a very easy level. It's possibly one of the easiest levels in the entire pack even though it's near the end of the third rank. Levels that require glitches can be seen in the same way, where the level is easy if you already know about the glitch needed but impossible otherwise if you don't know about it. I'm kind of indifferent about glitches, but I'm more towards the I don't like those kinds of levels, because in my view, the behavior of the skills should be predictable, not produce unexpected results.
Now, why do I believe there's an eventual limit on how hard a level can get? Because I feel like in levels where once you've seen a particular trick a few times, then you generally won't forget it and can more easily apply them in situations wherever they are applicable in hard levels. Also, this is where playing a lot of level packs really helps. Before I started playing custom level packs, I was able to beat the original games (Lemmings, ONML, and Holiday Lemmings 93 and 94). I first started playing custom level packs on Dos near the end of my high school career. At the time, I was able to complete most of the packs that I downloaded, but there were some that I wasn't able to until I eventually came back to them a few years later. Then, a couple of years after I finished college, I discovered a custom pack on Youtube, Dovelems. It was an intro video where rtw mentioned that the pack was for Lemmini, and the thing that really drew me to it was how it has directional select, i.e, hold down the left/right arrow keys so that you only assign the skill to a lemming facing left/right, respectively. After Dovelems, Pimolems was next, which is definitely harder than Dovelems. A level early in the Hurricane rank had me give up in frustration and so it was left in limbo for a couple of years. When I did come back to the pack a few years later, I was able to get past the level and proceeded to beat the entire pack.
After Pimolems, I decided to take on Lemmings Reunion by Icho. Along with working a job, it took me 3 months to complete. At the time, it was the hardest and largest level pack I had ever completed. Now both go to United as being the hardest and largest level pack I have finished. There were other packs that I took on before I decided to give United a try last year. Needless to say, I don't mind the difficult level packs, but as I have played almost nothing but United for the last several months, which I finally just finished a few days ago, I am completely burned out from all the very hard levels. So, I'm taking a temporary break from the pack before I will come back and resolve the remaining 7 levels that have been patched up by Icho. Around the time when I was finally on the final main rank of the pack, I had a lot of fears that eventually the pack would become too hard for me and therefore beat me, but because I managed to finish supposedly the hardest level pack there currently is, that's another reason why I think there's an upper limit on how hard a level can get.
So after all the United madness, I decided to take up Nepsterlems again, which is another really hard pack that's probably pretty close and up there along with United. Probably after if I do finish the pack, I will take on namida's Alpha Plus, which again is another very hard pack. Just when I thought I was going to just play packs on the easier side after being burned out by United. Guess I just want to get the hard packs out of the way first.
Ultimately, I'm also in a way a natural born puzzle solver. In particular, I'm a math puzzle kind of person. So, along with how I have gained so much experience from playing many level packs, these two combined is why I believe I'm able to take on hard packs that many people will otherwise not be able to complete. Generally, the most difficult part of hard levels is just finding the solution. Once found, then it's just a matter of carrying it out. While the solution may be difficult, the execution might not be or vice versa.