It's about 2 things
1. the principle. You get something, you pay for it.
2. the practicality. Youtube cannot run on fumes. It needs to generate funds from somewhere
If everyone decides to not take premium, it only incentivises youtube to harvest your data for a profit (yes, they're already doing it but that's not the point). Premium immediately pays for the product, and provides Youtube with the cash to run it's servers and pay it's content creators.
Not to mention, premium is pretty darned good, provides almost all the features and functionality that are available through other clients.
