Just leave it in water until roots form, then pot it up.

I grew a cutting in a water-filled beer bottle for more than 10 years! It wound around my living room. I kept the water topped off and added a drop of Miracle Gro fertilizer once a month and it did great.

Hi & welcome to GardenWeb!

Is there a submerged node? AFAIK, Philo can only make roots at a node.

There are many discussions on the house plants forum about fungus gnats. Generally, altering one's watering practices (which involves corrected perspective on when/how often plants need to be watered,) or the use of mosquito dunks, or combo of these, can be effective. Mosquito dunks are primarily used to keep mosquito larvae from being able to survive in bird baths, but also reportedly effective for treating potted plants.

Potting soil that has organic matter in it can have these critters in it, so you're right, repotting can be frustratingly ineffective. I hope this can help you prevent the need/thought to discard plants in the future!