I don't think that a Nihilego drop is necessarily unwarranted, but I truly can't believe that you're comparing it to Pokemon that it's entirely different from. Nihilego's drop should absolutely not be on the basis of how it compares to Mega Venusaur and Toxapex. It shares one similarity with those Pokemon in that it shares the Poison-typing, which can be used to remove Toxic Spikes. However, both Toxapex and Mega Venusaur are defensive Pokemon. Nihilego is a revenge killer with Choice Scarf that looks to potentially set up Toxic Spikes as it forces switches. You are fitting these Pokemon onto teams for entirely different reasons, so saying that Nihilego should drop due to competition it faces from either of those two Pokemon is entirely false and not valid in any sense of the word. I also take issue with you comparing Nihilego to Tyranitar, Mega Tyranitar, and Terrakion, since again, all of these Pokemon also function entirely differently from it, which immediately invalidates your suggestion that they give it competition (it's perfectly fine fitting two Pokemon of the same on a team if you can cover their weaknesses and/or they fulfill separate roles). Tyranitar is a physical wallbreaker, Terrakion is also a wallbreaker that seldom runs Choice Scarf in comparison to how often it sets up with SD, and Mega Tyranitar is a setup sweeper. None of these are roles that Nihilego possesses, which indicates that competition between it and any of the aforementioned Rock-types doesn't exist when it comes to fulfilling a team's specific role.Nihilego: C+ to C/C-
This mon is.... absolutely terrible right now. It's completely outclassed as a toxic spikes absorber by Mega-Venusaur and Toxapex. It's outclassed as an attacker in both types, being outclassed by Tyranitar, Mega-Tyranitar, and hell, even Terrakion by rock types, and Gengar, Scolipede and Mega Venusaur by poison type. Why would you ever use it in a metagame infested with Celesteela, Landorus, Mega-Scizor, Ash Greninja, and Toxapex?? I know it can run thunderbolt but it simply doesn't do enough to its many, many counters. It's terrible defense means you shouldn't expect it to take many, if any physical hits at all. While Getting both Stealth rocks and Toxic spikes is a nice niche, fitting Hazard removal is easier than ever nowadays, with popular pokemon like Landorus and Gliscor now having ways to remove hazards. It's excellent special bulk is hurt by it's weaknesses to common types like water, steel, and especially ground, which it is 4x weak to. It gets completely destroyed by Most steel types too, not having the moves to deal good enough damage to many of them. Examples of these Steel types are Magnezone, Heatran, Stakataka, Jirachi, and definitely Mega Scizor. It's scarf set can be appealing due to fast hazard setting, which is admittedly nice, but isn't nearly enough to justify being useful in OU, and definitely needs a drop.
Another thing I didn't particularly agree with in reading your nomination was that because of its Steel-type weakness, Nihilego should drop. Nihilego has been plagued with the inability to do anything to Steel-types since the beginning of the generation. Steel-types have been prominent in OU since then as well. Its vulnerability to Steel-types is already considered and is part of why it's in C+ to begin with. This isn't a newfound flaw in Nihilego that should be grounds for anything that happens to its rank any longer. The same is said when you bring up the presence of Pokemon like Lando, Celesteela, and Ash-Greninja hurting it, because those are Pokemon that have always been present in the meta that have already contributed to Nihilego's rank and shouldn't anymore unless said presence increases or decreases (and if anything, Celesteela has become less prominent, which benefits Nihilego).
If Nihilego should drop, it should be as a result of its niche becoming less appreciated in the current meta, not because it faces competition from Pokemon that aren't similar to it at all with the exception of sharing one type. You only brought up struggles of Nihilego that have existed for a long time, not struggles that are newfound or further contribute to its viability in the meta than putting it at C+. This post wasn't to say whether or not I agree with the nomination, since honestly Nihilego's viability isn't something I've put much thought to, but no one is actually getting anywhere in comparing Pokemon like Nihilego and Toxapex or Nihilego and Scolipede (absolutely irrelevant Poison-type anyway).