As a few reference points: the Phase One H25 which is now coming on 12 years old is still supported in the latest OS, latest desktops/laptops, and latest Capture One software. However as of Jan 1 of this year Phase One can no longer guarantee repairs on it (as you discussed some parts are simply no longer available); when they had to announce this they offered a temporary upgrade offer for H25 owners which valued their back at FAR above it's market value.
The original Phase One Powerphase scan back (not the later firewire model) which was based on SCSI was likewise end-of-life'd last year after 14 years. The ability to get SCSI parts (and the hardware/software to test/maintain them) forced this.
Likewise I think it's fully expected that in 12-15 years the IQ series will no longer be able to be repaired. For most buyers this is very acceptable, especially when compared to the alternatives of dSLRs. Tethering the Nikon D2X is no longer officially supported in Nikon's latest software, likewise the Canon 1Ds II and 5D Mark 1 are no longer supported tethered to EOS utility.
In the dSLR world it's not uncommon for a model to be considered entirely out-dated just 2-3 years after launch (e.g. Canon 5D to 5D2 was 3 years) and certainly 12-15 years for a dSLR might as well be an eon. For quick reference 9 years ago was the Canon 1Ds and 13 years ago was the <3 megapixel Nikon D1.
Or take computers for instance. The Power Mac G5 is only 9 years old and is nearly completely useless for nearly any mainstream work (cannot run recent versions of any major software or operating system).
But in comparison an H25 (nearly 12 years old) still holds it's own against a 5d Mark 3 in everything but portability/speed/ISO. As the H25 was sold primarily to studio shooters (e.g. product/still-life/catalog) it is still an absolutely viable camera system for those users and we still (even with the cessation of guaranteed repairs) have customers using them.
So yes, the point is very valid that this gear will simply not last
nearly as long as, for example, a well made 1970s view camera. However, in comparison to most electronics or digital cameras the medium format market tends to produce pretty long usable lifespans and I think that's the more relevant comparison.
If a client came to me and said they wanted a camera that would work the same in 35 years I'd sell him a well made film camera, a 1000 rolls of black and white film, a freezer, and a closet full of well sealed dry chemicals. I'm joking of course - I'd send him to B+H to buy it; we don't sell film
.