It's not reasonable for OEMs to expect people to keep spare parts on hand because they refuse to conform to the standard. NO ONE else does that.
You need to get out more. It's very common in industry, where, for example, production machines must keep going. The OEM will either include a spares pack in the quote or offer bonded stock on site (for a fee) and invoice if/when that stock is used.
More so for military supplies, where there will be lists for 7-day, 30-day, 6-month deployments, for example. That's the difference between mission-critical and your self-entitled customers.
If you call those weird slim power supplies that cap out at like 250 watts "standard" then I don't know what to tell you.
They're standard for a wide range of models and 250 W is fine if that's what the machine needs. The whole concept of a given OEM's range of offers is for commercial customers who buy or lease big inventory lists with OEM support. The OEM will have parts on 4 hours availability, if a customer pays for that kind of support.
250 W is what's required for the machine, as ordered. The OEM lists supported options, which may include one or two different graphics cards, that will fit within the PSU spec. If you want something more powerful, you start with a workstation, not a SFF chassis.
They never used to do this crap before ...
That's what happens when an industry becomes mature. Parts are optimised for the application and engineered down to what's needed, instead of drawing from a pool of standard components. There was a time when you had to add option cards for network, sound, run-of-the-mill graphics, mass storage ports, serial ports, parallel ports and more. Nowadays, those functions are mostly either integrated on the motherboard or just need a port connecting to a motherboard header. There are plenty of examples of processor, RAM and mass storage fully integrated on the motherboard, too, and not just in laptops.