OP is pretty much correct.
LON was introduced to give variety to the players, but in fact it's very limiting.
Most 2pc sets outdamage early LON build now. LON is also very power hungry, you need tonns of great gear to make it work. While even a crappy rolled set can easily compete with a well-rolled LON build. As of 2.4.1 sets were buffed greatly and LON is in obscurity once again
Like I said, you can't buff LoN as long as there are top tier LoN builds. You have to buff legendaries to improve other builds.
In the end, full ancient LoN is only a 15-20 GRift levels push. So builds have to work in T10 without LoN equipped.
The problem isn't necessarily damage multipliers (as they add a form of progression) it's multipliers that affect multiple skills/runes. LON can't be buffed outright because there are a couple of builds that are really powerful with it because the skills and supporting legendaries work well with each other. The issue is that a blanket x% to all damage, or to a subset of skills gets limited by the strongest skill. This is likely the reason EP was removed from Sunwuko, to allow the other skills to get a higher multiplier, but now it's basically limited by WoL, and thus TR and LTK are less likely to be used.
Actually, it's exactly the contrary. Damage multipliers help build diversity. Damage multipliers can be balanced by reducing or increasing those numbers. How would you balance Uliana 2.4? There is nothing quantitative about it except the damage of skills themselves. But if you change skills, you might change some other build that uses those skills as well, with it.
This is the main reason I dislike the decision to place these multipliers on sets, as they are all either flat damage bonuses (IK, Akkhan, Inna, LoN) or have a subset of skills they affect, because the skills themselves aren't balanced well with one another at the endgame, so there is always a best option. They may be balanced at level 50, when resource management or some other mechanic is still an issue (which is why they don't simply change base damage numbers), but once resource generation is no longer a problem, WoL is basically always better than LTK.
This is why I'd rather see legendaries like Madstone, and have sets with 2/4pc bonuses that are thematic and too powerful for a single slot, but not super OP, and then introduce some other system (charms, ability to invest in a skill for a multiplier, whatever) to handle the multiplier and thus the progression.
But more on topic, at the very least, the multipier on LoN needs differ by class, so that it has a chance at being competitive with sets in some form. As it stands right now, if there isn't a legendary or 2 that enhance a skill, even LoN can't save it, and most of the skills that do have leg support are already attached to a set.