Okay, with 1 part I do agree - and that is Void Staff. It's extremely powerful item if enemies have at least 60 MR (most enemies in late will have more than that).
However, Liandry? I think you are extremely overrating the item.
Against enemy with 2000 health and 100 MR you get a maximum of 59 damage per second and that's already with Void Staff. Also, the "maximum" damage I listed here can only be applied to shields, because you need to damage your enemy first in order to apply Liandry's passive (so, if enemy have a shield at max health, that's how much damage it will deal). On average it will deal 30 damage per second (and that's a lie too, because you mostly listed bursty mages, which will first burst and then Liandry will burn whatever is left - assume that on average Liandry will deal even less damage).
If we increase the health to 3000 (which seems about right for a regular fighter) and leave 100 MR intact, it will deal 44 damage per second on average, which is of course not that bad. But it's not "great and impossible to reach with any other item". Now it all depends on your playstyle and champion:
- You poke a lot?
- You have low scaling?
- You have higher dps than burst?
- You have a lot of Dots?
If the answer is yes to most question then you should most likely get Liandry (especially with Rylai Crystal scepter if you don't have a lot of CC). If the answer is no to most question then you should probably skip on Liandry.
Examples of good champions to buy liandry on: Swain (due to his DoTs and general high DPS), Rumble (his damage takes a lot of time, and he also have two slowing abilities), Karthus (high AoE dps, has a slow in kit, but still would benefit most from Rylai), Malzahar (all his abilities take a long time to deal damage, but he also needs Rylai in this case)
Examples of bad champions to buy liandry on: Fizz (high burst, generally good scalings), Veigar (also high burst champion with no slows), Syndra (has a slow, but Liandry won't deal much damage in the time needed for her burst to kill enemy champion), LeBlanc (another high burst champion)
Of course, those are just examples and I am sure there are games when you might need Liandry on Fizz or Veigar etc, and there are others where Liandry will give no real benefit even to Swain etc.
Anyway, it's a situational item that synergizes with some kits far greater than with others. Just keep that in mind.