D&D 5E King (for a day) of the Monsters: Rot Grub Swarm

[The Scythian]

Sorry, that's ridiculous. Four finger-sized maggots burrowing under someone skin "haven't actually done any meaningful damage yet" doesn't make sense.

It makes about as much sense as anything else involving hit points, which are a mostly abstracted measurement of a character's stamina, life force, health, and sustained damage. I'm confused what you actually want out of this thread, though. You started out complaining that the rot grub swarm is too dangerous, but now you're complaining that it doesn't do a bunch of damage up front, which would make it more dangerous.

The problem wasn't mine as a DM for not reminding one player that I hadn't told another player to deduct hit points.

It's not about reminding players of anything. It's about creating a picture of what's actually happening in the game world, using a combination of rules and more or less natural speech. However, I've already made my position clear about this, so we'll just have to agree to disagree here.

Sure they're slow, defenseless (outside a body), limited in senses, etc. That's why they don't make much sense just putting them out in the open where they can be avoided. The only use I see for them is having them hidden or on/near something the players want/need.

Well, if you continue to set up encounters with monsters in a way that nullifies their weaknesses or gives them distinct advantages, don't be surprised when those monsters turn out to be more dangerous than their CR would otherwise suggest.

A 3rd level Revivify brings the dead character back with 1 hp, but the grubs are still alive in its heart eating away.
A 5th level Reincarnate is a kind of big deal as it permanently changes the PC.
Resurrection works great, but it's a 7th level spell.

I'm pretty well aware of the options available for bringing characters back from the dead. (You left out raise dead. Also, while revivify won't remove rot grubs, it's not that hard to have another spellcaster use a Ready action to cast lesser restoration as a reaction or have a paladin use a Ready action to lay on hands as a reaction.) I've also been playing or running various versions of D&D for over 30 years. I've seen characters secure access to healing magic above their level in any number of ways--scrolls, donations or offering services to the local temple, etc. There are going to be scenarios in which characters can't bring party members back to life, but death doesn't automatically end a character.

I'm not sure why you are telling me to treat them as higher level CR; I used the 1/2 CR rot grubs against a party of five 5th level players.

I didn't tell you to treat them as a higher CR. I said that if you're running Tomb of Annihilation, you might want to consider treating monsters that can kill characters outright instead of merely downing them as more dangerous than their challenge ratings might otherwise indicate. Challenge ratings are a useful guide, but ultimately, they're only a guide. There's no substitute for considering a monster's statistics and abilities in the context of your game world, the adventure you're running, and the circumstances of the encounter.

I don't think continued discussion is going to get us anywhere, so we'll just have to agree to disagree. Good luck with the game you're running!