No because of how it works out, you are mixing chance of death from adventuring with chance of death from brutal execution by the gm. Part of running memorable fights is making players feel like their PCs could be in real danger even while the gm is holding back and throwing the fight without letting it look like they are. Coleville has a video about what to do after a PC gets killed (I think?)where he says something along the lines of "I'm not trying to kill you, these [goblins?] Are trying to kill you."Shouldn't terrible PC saves be a good thing for all those DMs who complain PC are unkillable? Just target their weak save!
That statement is a critical change creating a brick wall there now is that the gm has a razor"a edge between looking like nobody is trying to kill the PCs and looking like the PCs have annoyed an angry vengeful god intent on crushing a PC. The second feeling there is because When things go sideways a question from that same video "what could we have done differently to avoid that" is now rarely a question that has any good answers other than rebuild everyone's character for more extreme minmaxing.
Except there very much is a "standardized power level" two of them. Those power levels are curb stomp and rocks fall. The tools to widen the razor's edge thin line between them have been removed designed against or left with a design space that almost guarantees they will push things far to the other side of the lineI understand your scenario here and understand your complaints about this situation... but I will tell you from my perspective why this complaint doesn't seem to hold much weight for me. Because it assumes that the game itself doesn't give the players/DM any choice in the matter. That they are just stuck having to face off against this naga again in a few levels and be just as bad at facing it as they were several months previous.
But that's not true. The DM has chosen to put their players up against this naga again. The DM has known going in that their players are going to have just as bad a time of it now (even at their higher level) as they did last time. And it seems to me that that's exactly the point. The DM wanting to throw a difficult encounter against the group, and where the party just happens to have that one achilles heel against them regardless of how strong they have gotten through the campaign.
D&D is not a game where there is a standardized "power level" for every single type of creature in the game, and as the PCs gain power, every creature's power begins to fall away equally. Like a naga that was a certain power level when the party was 3rd level needs to be demonstrably less powerful when the group is at 9th. That's not at all how this game has been designed. And we know this because the game is set up to allow us to build creatures at whatever power level we wish. Default kobolds from the MM are CR 1/8... but any DM out there can just build a kobold using the PC rules and thus throw out an 18th level Fighter kobold against the party if they want. So the party had a certain difficulty against kobolds at the beginning on the campaign, and had the same difficulty against some kobolds three years and 15 levels later. Or you have twelve different types of zombie, each of them at a different CR and the party will face those zombies over and over again, all of them always seeming to be a challenge. So we can't say there is or should be an expectation of monster power level depending on PC level. That just doesn't happen. Players can and will run up against all manner of power level for every single creature out there.
Thus to my mind... the fact that this particular naga does not get weaker against the party across the board (but rather only in certain ways) is nothing that seems wrong. Facing off against a monster that gains or loses differing levels of power in their abilities depending on which part of the monster's features we are talking about seems completely normal.
Someone @James Gasik (?) Brought up the idea of making+resist gear give proficiency bonus to a save and allow the save to be swapped on a long rest. That simple bandage wouldn't really solve the serious problem. Firstly it would make them into perceived required one &done gear items with no need to upgrade them later or find others. Secondly is the saves in 5e itself... Instead of having one good save one ok save and one crap save across three important primary saves, 5e PCs have one good save in an important primary save plus one good save in an almost entirely useless save and four crap saves in almost entirely useless primary saves. That ok save was important because items could make it good just as they could make a bad save ok. Wotc needs to go back to three saves and stop pretending rock paper scissors lizard dynamite PLUSONE MORE with saves is doing anything other than harmfully diluting saves to a pointless degree given the other mechanics interacting with them.