Paid versions of the agent are primarily targeted at those businesses who needs technical support for agents. If you don't need technical support, then indeed you can just stay on the free version. We have no plans to discontinue free agents, and will only keep enhancing those.
Mixing free and paid agents is not allowed because this would enable customers to potentially roll hundreds of free agents into B&R and still get full support for the overall solution due to the presence of a few paid agents. And of course, supporting a B&R server that is teared apart by hundreds of free agents has substantially higher cost for Veeam than supporting a B&R server targeted by just a few paid agents... so the price of those few paid agents simply won't cover our support costs for such a deployment. Hope it makes sense.