I am at a loss to figure out what is going on with this orchid. It had a mealy bug problem, I cut off the flower spike, repotted it, and after a while got my first keiki ever growing from the flower spike and a lot of new root growth.
Now all of a sudden, the leaves are turning yellow.
Is this a systemic problem that will eventually work it's way up the orchid or are these the old leaves that were unhealthy and are dying off?

Does the pot that you have selected have drainage holes? I am assuming that it does.

The first think my experience tells me to suggest to you is to remove the keikie! Yes, that might seem harsh but your Phalaenopsis is in trouble. The root system became compromised by staying in wet, decomposing media for too long. One year is great, but repotting every two years is all right. Those roots need oxygen to breathe and survive. Without it they die. When the viable root system is not as large as it should be, the plant looks shriveled. A keikie can be the plants reaction to try and save itself long term. When you finally repot a Phalaenopsis with a compromised root system, it typically takes 4-6 weeks to form new roots. Then in time those roots absorb more water and the leaves plump up.

The reason you want to remove the keikie is because you want the plant to make new roots and promote better health. You don't want the orchid to split its energy or resources on a keikie. The baby orchid ends up compromising the health of the mother orchid.

Remove the dying leaves, they will never return to good health. The mealybug attack coupled with delayed repotting has lead to the plants poor health. No disease is to blame.

Thank you! You figured it out exactly!
I was displaying my orchid on top of this vase filled with rocks and moss. After I posted my message, I noticed the moss had started growing! I realized that I had created a hothouse under the roots and no air was escaping. Back to the internet and I learned orchids roots need air and I had been suffocating the orchid. I will cut off the keiki as hard as it is!
What is strange is that the person I follow on YouTube repots her orchids in plastic cups with no drainage holes and they thrive. How is that possible?
I am attaching a photo of what my orchid was sitting on.

Well successful orchid growing can be accomplished many ways. We don't all do it the same.

Most orchids are what are known as epiphytes. And as such, in the wild they grow on something be it a tree trunk, a limb, or whatever. It is for support only. Their roots need access to oxygen. In soil, they would suffocate and die.
Orchids for beginners include Phalaenopsis, Paphiopedilums, Oncidiums, and Cattleyas.

If you would like to get access to good information visit the American Orchid Society's website @ aos.org! At the top part of the webpage is a 'membership' tab. When it opens there should be a bullet point for CULTURE SHEETS. These are printable and downloadable information sheets. Get the one on Phalaenopsis.
Also for tons of information check out the St. Augustine Orchid Society in Florida. Just browse the site, there is tons of good information there.

And again, anyone can post videos about orchids. They may or may not know what they are talking about. But I am here to help you so welcome to the Orchid Forums at NGA. In fact a few dozen good orchid people are here. Welcome from all of us!!!
Me personally, I have been growing for 46 years.