I forgot to say that my mother in law advised me to cut the whole plant down to the ground and it will come back next year. I don't trust her advise. She buys the orchids, keeps them in flower, then throws them away.

I only ask because I thought that this plant deserves to survive, she wants to live so badly, she even gave me new growth... ( the new growth are about 3 inches tall by now)

Don't listen to your mother-in-law! If you do as she recommends, the roots will have nothing to provide nourishment. Even the leafless backbulbs store energy for the plant.
Cymbidiums are pretty hardy. If they grow well, the time will come to divide the plant. (The alternative is to keep it whole as a "specimin" plant, but it can be challenging to repot and give it enough room to grow but not so much that it's swimming (read rotting) in bark.) If you're using 18 litres of medium, it sounds like it's time to divide. If you do it now and properly, you can get blooms next year. They don't need to be root-bound to flower.

A group of four or five pseudobulbs will provide a viable division. When I first was instructed in this, I was told to take a large knife and just cut down at the chosen division point, all the way to the bottom of the root ball. Remove as much bark as you can, and repot. Put the leafless (old) pseudobulbs close against the edge of the pot, and give the opposite side (with new growths) at least 1-1/2-2 inches of bark between growths and edge of pot.
Since I made divisions like that, I thought I got more sensitive and the last few times I actually soaked the root ball and gently untangled all the roots, and divided by pulling apart. This is very time consuming, and a few roots invariably get broken. Although I think this must have been less stressful for the plants, the recovery and resulting divisions seem to be the same.
Ann

DonÂt quite understand what your mother-in law means by cutting it down to the ground. A pseudobulb has a limited number of leaves, so if you cut back all the leaves, they wonÂt come back the next year. If you are potting up (which is what you have done) or if you are dividing really depends on how much space you have and how big a plant you can physically handle. As the previous poster mentioned, sooner or later you will have to divide your plant because cymbidiums just keep growing.
I am not sure if the flowering of your plant will be negatively affected because you have 4 inches of space between the plant and the side of the pot. Personally, I believe there are a lot of other factors such as light levels, temperature drops at night and fertilizer that affect flowering to a much greater extent than the size of the pot. Every cymbidium is different and it will take a bit of time and experience to figure out what growing condition is best for your plant.
Good luck