I am building my personal private cloud. I am considering using second hand dell optiplexes as worker nodes, but they only have 1 NIC and I'd need a contraption like this for my redundant network.
Then this wish came to my mind. Theoretically, such a one box solution could be faster than gigabit too.
Sure, but you forgot about reusing perfectly good older 1gbit equipment with sufficient ports to do nice 4gbit bonds. I have been doing that with 4 port Intel NIC PCIe expansion cards for a while on those servers with free slots, but on those thin clients re-purposed as servers that is usually not the case.