Abrasive cost and maintenance (because high pressure water lines are quite needy) are the biggest expenses.

Lets say you only cut steel. Then the steel particles could be filtered out with magnets. I suppose its much more common to want to work with steel than glass or ceramics.

Nobody I've heard of recycles garnet. It is considered part of the cost of operation.

If you only cut steel you should be buying a plasma cutter.

This 100%. Water jet is great when you need to cut things that don't cut well any other way (tiles, glass,stainless steel, hardened tool steel). Otherwise, plasma, laser, router, mill, etc are all nicer and easier to work with.

Yeah, I'd love to have a water jet for making tools, but I've learned to live with the fact that these kind of things are better owned by someone else.

idk, plasma leaves a lot of slag and depending on the application requires substantial finishing work. mill is fine but has a limited work area (like this waterjet), and at least in my shop, cutter lifetimes aren't great. Thats probably because i don't have the best processes. Also holding down sheets well enough is a substantial hassle unless you're set up for a particular run. Metal routers are really noisy and are arguably more of a hassle than mills.

but I totally agree with your last point, its less convenient, but doing small to medium runs though a local water jet shop is pretty cost effective. Its mildly pricy but they will source standard material for you, and you pick up clean parts a day or two later. even ignoring maintenance - dealing with the up front cost, the consumable cost, and renting enough space to keep the thing is a pretty serious commitment. I have free access to a 4x8 cnc plasma gantry and I still send out quite a bit of work to the water jet place.