I’ll do my best. CUPS is quite opaque to me 
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From my understanding, a lot of printers require filters to convert postscript output to whatever raster format the printer requires. Some printers accept postscript directly, some work well with open source filters and others need proprietary filter blobs to talk to their hardware.
I have a Samsung and a Brother printer. Neither produces useful output without the blobs. The unified Samsung driver packs 64 and 32 bit binaries, which is fine. The Brother one only has 32 bit.
As you’ll know, many printer manufacturers don’t support Linux at all. From my limited experience, I haven’t seen a printer driver filter with complex dependencies, so a limited i386 runtime may well be enough to keep them going. I wouldn’t be optimistic that new 64 bit drivers would be released for old hardware, but Canonical might have the clout to swing this.
Of course, for many years it has been convenient for hardware manufacturers to release i386 binaries for their userland, as these would run on 64 bit and 32 bit installations. As that hardware dies, hopefully the next generation will come with proper 64 bit support. But some stuff like those Brother printers are robust, cheap to run and work well. I think they’ll be around for many years to come.