This is my employer's support model, and it works pretty well - all customers get unlimited ticket/email support, and you can upgrade to phone and live chat support for a percentage of your monthly bill.

It's like the saying that 20% of your customers use 80% of your support. This way you can incentivize those customers to pay a premium for even more of the support package they obviously want. Everyone else can have the minimal support interaction they prefer.

Although if I encountered that as a customer I would be suspicious that the free support is being made deliberately bad to artificially drive revenue of the premium support team.