Keep the hacksaw in the drawer. Making one width of crank is a cost saving for both Shimano and the consumer. If they made two models, they would have to keep an inventory of both as well as machining for both. Twice the cost. Spacers are cheap and effective as well as allowing for a wider range of bikes the crank can fit on. Adjustment for chain line is also easier and cheaper for the consumer. Internal bottom brackets need to come in a variety of widths and it’s a bit of a trial and error method to find the right one if, for example, you want to use a mountain crank on a road bike. Mountain bike cranks tend to actually use narrower bottom brackets than road bikes because the crank curves inward at the spindle where a road crank doesn’t.

This is all fairly easy to do at home, by the way. The bottom bracket tool is relatively cheap and easy to use. The fastening bolts on the crank make it so that all you really need to take the crank off is a 5mm allen. The process is far less involved than removing an internal bearing bottom bracket. As a bonus, you aren’t without your bike for 2 weeks while it waits in a shop.