PLA isn't brittle. We've used it with great success for tooling fixtures at my work where we manufacture aircraft parts.
When compared to ABS it's not quite as strong. mostly because ABS has a bit of a rubbery property to it, but most of my PLA prints you could throw at the wall hard and they'd come out unscathed.
I use PLA because it's more than strong enough for these kinds of things AND it has a higher quality finish and is more dimensionally accurate than any other material. Since most of these things serve a decorative purpose as much as a protective purpse choosing a material that will look nice and fit well is ever much as important as the protective aspects.
There are a handfull of parts where I make them in ABS and that are things such as the Sega Saturn battery door. since the part needs to bend under normal use ABS is more suited for that. Similarly I'll likely make the CPS2 buttons out of ABS for the same reason. ABS shrinks when cooling, this is extremely difficult to control for so parts have more difficulty fitting properly, but it's worth this annoyance in some applications for the bending properties. ABS is also better for higher temps so for instance I used it when designing headlight brackets for my car since it's going to be in a very hot environment.
For cartridges the PLA material is actually very well suited. It has some of the best wear resistance properties of any of the filaments available, it's far less likely to wear down over time compared to other materials so for use in a device that sees friction against other surfaces regularly it's arguably the best material for the job.
90% FMD 3D prints use those two materials, there are some new materials that blend the two to give you basically something that's half way between the two (a little stronger and higher temp than PLA but a little better dimensionally than ABS) but I find that's typically more suited for people who just want to buy one material that works "pretty good" for everything rather than picking a particular material thats suited for the particular part.
Nylon is another option, it wears better than PLA, is stronger than ABS and and handle higher temps than ABS, however it's nearly impossible to print with, dimensions are extremely difficult to nail down since it has a high propensity to warp and the finish on the final part usually looks like garbage. Not to mention the color options are mostly just "natural" which is like a yellowly, slightly translucent white. About the only application it's worth using Nylon for is if you're making something like custom gears that will see lots of constant wear, high stresses and heat.