To start I think we have to look at this question at high levels (as the OP kicks this off with a high level dragon conversation).
I do think high level monster design is still flawed even in 2024. The game ramped up the math but it still doesn't respect how fundamentally different high level dnd is.
To that end, I do think legendary struggles to do its job. As noted, damage values are not high enough to deliver proper threat to a high level party. The issue here is that high damage can lead to swings where players take extra damage and die.... but at high levels thats really not a concern (tons of ways to avoid death or just resurrect).
Another example, the game assumes that one high level damage attack is equivalent to 2 or 3 attacks if they all do the same DPR. This is absolutely not true at higher levels, if you are doing an alpha strike, it has to do a RIDICULOUS amount of damage to deliver credible death threat, as compared to a 3 attack combo that can blow through a players death saving throws and deliver damage. While LAs in theory let a monster deliver damage in response to a player, because these single buckets of damage or often relatively low they don't deliver the same threat as a focused hit with multiple attacks.
A rethink of high level monster design I do think is useful. For example the recent discussion about "hitboxes".... which invokes the classic Final Fantasy idea that boss monsters often go through "phases". You beat phase 1 and then the boss moves to phase 2, with all conditions removed and at full health (no matter how much nova the phase 1 monster takes). Such a design bypasses a lot of fundamental powers of high levels PCs and is a very effective design that is underutilized in 5e imo.