I don't think many of your fears are warranted.
Even if we start talking about firmware updates, many of them are very simple and only require you to use associated PC software to download and then install the firmware updates. In many cases, firmware updates aren't like patches for video games - camera manufacturers are not in the business of making half-assed firmware full of bugs and glitches.
Usually, firmware updates exist to add features which weren't originally part of the camera (enhanced autofocus, colour profiles, adding support for new accessories and/or lenses which weren't available when the camera was first released).
Firmware updates are also not frequent. You'll maybe get 1-2 a year, sometimes less often. Though, in some cases, the firmware updates may be produced upon feedback from industry users, such as professional photographers, videographers and cinematographers, and can feature improvements to how the video compression works or adds support for external video recording.
In terms of obsolescence? The only thing likely to become obsolete in a digital ILC is going to be the image processor that spits out jpg files. This is why many photographers prefer shooting in raw image formats, which save the image sensor data in a proprietary format. This raw file can then be edited in software like Adobe Lightroom, Capture One, Luminar 4/AI, Rawtherapee, Affinity Photo or Darktable. A 12 year old 21mp DSLR might take jpgs that look like total garbage today, but if you take the .cr2 raw and take it to a raw editing program, you can still produce high quality jpgs or tiff files which can be used to still make very high quality prints.
The "worst" kind of obsolescence exists in video features, particularly as our consumption of high res video increases. But if you're balancing film vs digital, this is a non-issue.