There's a number of alternatives for extra i/o - it really comes down to

1/cost - it may be cheaper going to a larger micro for more i/o.

2/ performance - most applications don't need to update the i/o at high speed - so the 4021/4094 or 74hc165/74hc595 techniques are very popular, cheap and easy to expand. I use these techniques for switches and relays. I update at 10 times/second which is more than adequate for a lot of control apps.

3/ size. Weird-ass ics make for a compact solution but can be difficult to source in small quantities and/or at a reasonable price.

Other common methods are i2c,spi,1wire, via latches/buffers off the bus. If you're a little more ambitious, CPLDs and FPGAs are a good choice for applications where you need specific interface logic for high speed or where you need to squeeze a lot into a small space.

It's your job as the designer to trade off the various options to come to a solution.