Can Oncidium survive with one remaining pseudobulb?
 

I have a Twinkle which has been through quite a bit of neglect. After cutting away rotting bulbs, i am left with one bulb and that's it. Is it possible to recover and one day thrive (in many years)?

Yes, it is possible. Does the bulb have leaves? Roots? If not, it may have enough strength to generate a new growth, which will bring roots along with it. A picture would help get you sensible advice.

Even if there's a 0.0000001% chance, it's still possible. So, yes!

Personally I wouldn't worry about it, but that's a judgement call. Twinkles are relatively easy to find and inexpensive, so if I still wanted one I'd probably just go get a new one and save myself the stress. Not everyone shares this philosophy.

If you want to attempt to save it, look up sphag and bag. The concept is basically to set the bulb on a moistened bit of moss down into a plastic bag and keep it in a warm shaded place for months or years until you see a new growth and new roots or it dies. Depending on how it goes, proceed accordingly (i.e. repot or discard).

It has one leaf! But one is better than none haha. It has some decently poor roots, but they're there. It's in semi-hydro not and being meticulously watched after so I'm hoping for a lot more root growth.

With one leaf, it can photosynthesize, so your chances of getting a new bulb developing are much increased. Warmth, constant moisture, and modest light will improve your chances. If it has been in S/H all along, keep it there and give it the best culture you can. If it is new to S/H, this is not the time to make the transfer. You would do better to put it in a moisture-retentive mix until you get new growth. The roots on the new growth will adapt to the S/H conditions.