If you have the hardware then it's quite viable option and with ROS now being able to run containers, it's quite interesting option.
In this case it would make much more sense to do it the opposite: run "plain" linux on bare metal (more likely to get all HW supported, more likely to even get basic OS to boot), then run some decent docker implementation (the one in ROS is frankly a PITA in comparison) and a hypervisor running a CHR. Some more standard services can be run natively on base OS - if that was ROS, most services would require running containers.
As to why woukd one need L6 license over L4 ... how many PPPoE clients is your router going to serve? If more than 500, then you need L6. Ditto for question about number of OVPN tunnels, or number of hotspot users, etc. I still think that the table is pretty much clear and you just have to consider all the table items and anwser yourself a question which license level would suffice to cover needs about that particular item. After you are done with the list, pick highest level needed for any of items and you'll have your final answer. If you decide to go with CHR, then selection process is much simpler, you only have to decide about maximum port throughput ... if your hardware has only 1Gbps ports, then you need P1 license. Etc. Even if your hardware comes with faster ports but you know you won't need it (e.g. your server comes with 10Gbps ports, but the rest of your network is 1Gbps or your internet speed is less than 1Gbps or ...), then you can purchase "slower license", ROS will throttle throughput down to licensed speed but otherwise it'll support all functions.