... or a nicely modular design, with lots of PCI-express slots, so you can add the ports as you need them 4 or more at a time.
There are plenty of such examples of embedded SBC with PCI expansion slots, but not router-optimised ones with PCI-E which is really needed to avoid PCI-bottleneck at GBE speeds.
Any case, the CPU will be limiting with the ~3Gbps, which for a high-port-count-router may or may not be a problem depending on the application. AFAIK this is the case with most software-driven routers.
High-port-count devices you see like HP Procurve are generally "layer3 switches" which use hardware ASIC to run the protocols at full line rate. Other large routers like Juniper etc have complex core fabric and "network processors" on line cards, which is inherently expensive.
Perhaps a "stacking port" concept could be the answer - put multiple routers in a single enclosure, all wired together with some inter-board high speed bus (PCI-E again?) - and then the software could scale up to allow higher aggregate throughput with the per-board ~3Gbps of processing power per 4 interfaces. Don't know how that would work with the RouterOS architecture or roadmap - but would meet the demands of power users nicely, doesn't add unneccesary cost to a smaller router, and adds some inherent redundancy too in case a single board failed.
CableFree - Wireless Excellence - Microwave, E-band Radios, Free Space Optics, High performance Radios & Routers