Have microATX motherboards and cases become obsolete, or are they just impractical for today's system builds?

Whether it is obsolete or not is dependent on the processor. The small form factor of the microATX, with its limit of 2 PCI slots is not a limitation in itself. It looks like most peripherals are going USB connect. Whether they start making this motherboard that will accept PCIe graphic cards will be the determination of whether this size of board stays available that much longer. My gut feel is computers are going convergence with television/telephone/computer as a single appliance and that's going to require a TV-out connection.