alt Hacker NewsNot true. You are confusing "digital" with "microprocessor". You wouldn't be able to do any single-chip microprocessor, of course, but something like 74181 is very doable at this scale, and building a 1970s-era computer out of a few dozen of these is something enthusiasts still do. The main problem isn't logic, it's memory - memory needs density (thin film magnetics anyone?).
Then, of course, if by "useful" you mean "commercially viable", it is indeed not going to be competitive against either TSMC or your local 500nm foundry ever.
A CPU made with ALUs like 74181 would take alone a PCB of ATX or eATX size densely populated with integrated circuits and consuming much more power than an entire computer consumes today, while being slower than a tiny microcontroller with a cost of less than a dollar, which also includes enough memory for a practical application.
I call such a CPU as not useful.
It can be a very useful experience to design such a CPU, but you can simulate the design in a logic simulator and you gain nothing by building it.
As a valuable computer building experience, it is more useful to use much older components than digital integrated circuits, where you can see nothing without special instruments, e.g. you can build interesting computer blocks, like adders, registers, counters etc., made with electromechanical relays or with neon glow lamps, where you can see with your eyes how they function.