Sorry, no idea. What you are doing sounds good. It does not look very old beyound maybe 4 years. But sounds like it may have been in the dark for many years untill now. It should start to pick up. I hate to tell you to take it out of the pot and check the roots out because in it's weaken state it may not survive it.
Waiting to see what else advise you get here.

I certainly agree, but what kind of soil are you using, does drain fast, is there a hole in the bottom of the pot. It looks like the roots are rotting. Perhaps too much sun for that species, not enough air circulation? Once the rhizomes (roots) are developed it needs very little water about once a month a good deep soaking then go on a month trip and leave it alone it may need a rest. This is common in the winter but this time of the year.

I keep several tiny snake plants in a windowsill where they get direct afternoon sunlight. They get a shriveled up, wrinkly texture when I've left them dry for too long. Is it very warm with direct sunlight where this plant is sitting? It might be baking. I'm no expert, but I'd probably stick my hands down in that soil and feel around the roots a bit. At least then you would know if it's dry or wet. (I did this once with an ailing lady palm and discovered a maggot under my fingernail afterward... I repotted that plant)

Ugh gross.

Tinkertoy, are any of the leaves darkened and mushy to the touch? If so, cut them off to the healthy tissue or even to the bottom with a sterilized knife, scissors or a shaving blade. My understanding is that the leaves that are bendy will not right themselves.

The plant looks pretty sad - I would carefully unpot and see if there is any root rot. I would say repotting is in order. As much as it is a shock to the plant, it's a much smaller shock than this plant seems to be experiencing right now. Sanses are tough, and you may very well be able to save this one.

The soil does look very fine and peaty. Give it a "cactus and succulents" potting mix if you are going to repot. It'll feel better.

After repotting, you might want to stake the taller leaves to help them stay upright, especially if the roots are not extensive - Sanses often have small root systems.
Let us know how it all turns out.

Tinker, the soil needs to dry more thoroughly than just the surface. Wait until the pot feels much lighter. If it already feels weightless, it may be so dry it's hydrophobic. Which do you think it is? Too dry or too wet? Looks like the same kind I have that likes to bake in the sun outside all day (when it's warm enough,) S. trifasciata, but hard to tell for sure. If it is, the more sun, the more distinct the variegations, light green for the green parts.

Sans roots can be quite extensive, it's how they make pups.