They have active economic reason not to do this. The first set of patches not from an official source would be to disable code that enforces using their expensive consumables instead of cheaper options.
Remember that most manufacturers aren't printer companies. They're ink/toner companies.
Printers haven't substantially changed for years, all the (literally) thousands of new models being churned out are presumably to avoid anyone bring able to standardise on a third-party work-alike ecosystem that would remove the barrier to entry. No one needs the HP CrapJet 2321e to be developed except HP. Most new features (say WiFi or Dropbox printing) could really handled with software or at worst an upgradable control module[1], without redesigning all the mechanics.
Currently, if you want to sell your own ink or toner, you first have to build a printer and defend it from undercutters who would like to not have to pay to build a printer first. Or, if you are the undercutters, you have to constantly play catch up as the printer manufacturers change the products to make your job harder.
An open printer hardware platform is what's required, à la Framework or Prusa 3D printers. The non-printer hardware (ports, WiFi, screen, CPU, power) is almost trivial now. This wouldn't actually be that hard: the data rates are low and the mechanical issues are mostly solved. But without that, you're always going to be making reversed firmware for last year's printers and HP can make the CrapJet 2322 with "new improved ultrasecure" features faster than you can break them, and now you're behind and the 2322 is already being replaced with the 2322a and your users can't even buy the models you support except on eBay.
The other problem is that the incumbents have thoroughly mined the path with overlapping fields of patents, so if you tried it, they'd hammer you into the ground with lawyers.
[1]: basically a slot for an SBC with a standard connector to the printer hardware. Even if you need a new control board because you want a 5G modem or something, you can still use the same printer mechanism.
Community support & bugfixes. There's no real downside either, all printers that exist today are crap so it's not like they differentiate on firmware quality
They don't want long term support. They want printers to get old, drivers buggy and broken, and get thrown away so you'll get a new one with updated DRM. Ideally before the third party suppliers can really dial it in.
Even better if you have to throw away a few extra cartridges with the scrap machine because they don't fit the new one.
Just to be that guy: iOS/Mac users don’t have to worry about printer drivers or older printers working with newer devices. As long as you buy an AirPrint compatible printer. An iPad from 2010 that hasn’t been updated since 2011, can still print to a brand new AirPrint compatible printer. An AirPrint compatible printer bought in 2010 will automatically be recognized by a 2023 Mac or iOS device.
Your other point about DRM ink/toner is orthogonal to that.