I think that it is history....I see no buds on the trunk that could take over. I would start over.

Oh man it sucks to just give up on it. Is there really nothing I can try?

Well, since there's no big loss if it doesn't work, how about re-potting it in a very well draining mix, and putting it outside for more light.

If it's been inside a long time, set it out in the sun slowly, like a couple hours the first day, then back in a shady spot (but still outside) more hours the second and so on over the course of a week or so. That way you'll not shock it or sunburn the leaves.

Also, don't fertilize after re-potting until you see new growth, then use a good liquid citrus food at half strength.

Avocados don't need a lot of humidity, they like it just fine but they don't NEED it. If they did they would not grow in areas like Southern California, which is basically a desert. So I wouldn't worry about that if I were you.

And I would snip off that brown tip just past it and into the green trunk. I've found brown tips like that just keep going unless you cut them off.

It might still come back, imo, 50-50% chance.

The fate of this plant is all in the roots now.If the plant had a LOT of leaves which were taking up the water in the soil, then your soil mix and the tall stem made sense, but right now you must not let the roots rot since this cycle has effectively shut down.

In fact, I suspect that the problem started with the roots, most-likely too much water retention in the soil.

To prevent the rot you must dry up the soil, but not for too long... add just enough water to encourage those roots to regrow. It's a balancing act.

The green stem is still photosynthesizing, which feeds the roots, so you'll want sun, especially to keep the soil from being wet.

With new root growth you'll see some budding on the stem, and then you'll be in the clear.