I have occasional use for high ~3000psi pressure,
Do tell what for ?

I was wondering when someone was going to ask that one. Heh.
First of all, I take safety very serious, and I hope I've made that point clear to everyone who has posted here. Improperly set up HPA is extremely dangerous and can result in explosions, various types of missiles, and high velocity shrapnel. So, yes, I take related safety seriously. But like any motor vehicle, HPA is safe when properly and responsibly used. A crazy driver can get on the highway and cause massive property damage, injury, and death, similar to improperly set up HPA. The big difference is the danger involved with a crazy driver is very obvious, but the dangers involved with HPA are NOT obvious and require not only good sense, but also experience in identifying the danger points. When it comes to the safety of others, I am good at exercising good sense, but I lack sufficient experience working with HPA to design a guaranteed-safe system. Some required safety bits and pieces are obvious, others are not.
Some posters on this thread have shared some of their valuable experience, and I greatly appreciate that (and so do my neighbors). Please be assured I am applying that good advice to make sure all required safety mechanisms are in place and tested as much as feasibly possibly long before even plugging in a HP compressor.
To answer the actual question: Well, I have several uses for high pressure, and that is assuming I do not get back into SCUBA any time soon. I still have a closet full of underwater gear that I should probably either use or get rid of... Spring cleaning for another day. But I digress. I've been trying to get my nephews interested in building a robot with me, since they speak some new fangled language my older ears do not understand, figured robotics might be a good rosetta stone so to speak. I'm not going to give them any HP components, but I have some ideas I want to try out adding pneumatics to a mobile platform. I also have a few friends that are into guns and go to an outdoor range pretty often. I'm not really into the whole firearms thing, but picking up a high powered air rifle to "me too" without compromising on the "fire"arm part would be good I'm thinking. The main reason for high pressure, and my immediate need for air compressors in general is adding pneumatic part development capabilities to my lab. I'm currently working on several projects for 3DP pneumatic parts and I need several different pneumatic power rails (aka low pressure manifold with several low pressure regulators putting out different pressures). Plastic has been used to convey pressurized material for a long time. Plain old PVC can safely contain a substantial amount of pressure, with >1000psi rupture point. The single biggest problem (due to safety) with 3DP pneumatic parts is the unpredictability of the rupture point, and where in your design the rupture will occur. The second biggest problem with 3DP for pneumatics is very poor dimensional accuracy and repeatability, but much of this can be overcome using o rings, normal metal springs, and other non-printed bits and pieces. It is not a small challenge to come up with
practical 3DP pneumatic designs. But being a responsible designer requires significant testing, including destructive testing. I have an outdoor steel enclosure with steel grating over vents that is suitable for failure testing parts with very thin plastic walls, but even that basic testing will require a little more pressure than a typical consumer air compressor can produce. Doing failure testing of parts with thicker walls will require a trip to an outdoor gun range or similar location for obvious safety reasons. There is no practical way to test the reliability and safety of a design without destructive testing in a safe and controlled manner, like placing the DUT in a vented steel enclosure and then over-pressurizing the thing until it fails. I'm sure I'll come up with more uses for HP as time goes on.
To put my HPA "system" in perspective, it's basically a small 350mm x 175mm x 360mm HP compressor, a pony sized HP tank, plus all the necessary bits and pieces for safety. Compressor and tank will be located directly next to each other, no long haul HP piping. No matter what, I will still need an adjustable regulator for HP to LP so destructive tests can start at very low pressure and brought up slowly until DUT rupture. A pony tank should have enough capacity for a single destructive test, but if also using "system" for LP, will probably use a normal sized scuba tank, but in no scenario will I have larger than a 120 cuft scuba tank (like I used to own).
As far as trying to use an HPA system also for LP: The thought is simply that once I have a
small HPA system designed and SAFELY built, not much else is needed to also use it for LP. Since my home is space constrained, minimizing square footage by using a single HP compressor with HP tank of conservative size instead of both an HP and separate LP compressor with comparatively very large LP tank would be a very good thing, although not a 100% requirement (more like 95% required).
It is extremely inefficient to use HP for LP, but no reason you can't do it. The tricky bit here is not how to use HPA for everything, but how to design and construct the HPA with all safety concerns addressed, with or without the LP step down. I owned and used HP scuba tanks before, even had them turned and certified for nitrox, but setting up that same tank for continuous use and connection plus the compressor (for attended use only), has significantly different safety needs that I am not sufficiently familiar with yet.