15x units are ALL expected to be updated for disability rules which apply from 2020 - by which time they will already be over thirty years old; but they were built well, to solid designs, and the ROSCOs seem confident that they will last long enough to justify the huge cost of new toilets etc.
Many first generation DMU/DEMUs outlived their official use-by date for many years, but contrary to popular belief, many didn't.
The 1938 stock on the IOW speaks for itself, of course - I thought they'd grab a few of the 1972 units that the Vic had added to their 1967 base, but not a bit of it.
As for the pacers, they were expected to last about 20 years (25 years ago), and were built to a much lower specification than, say the 15x units - let alone 180s or HSTs, which were built like tanks.
Life extension for 14x units would cost much more than any other unit (because the difference between build quality and 2020 requirement is greatest), and the value of any extension would be much lower than any other unit, basically because they're cr*p.
So far, I have heard no suggestion (outside these forums) that anyone should even spend a penny on painting the things. I don't think I'm alone in suggesting that they should never have been built; it was a false economy.
... and all of this in the context of huge numbers of DMU/DEMU being released by electrification. Even the 16x units can (and will) move beyond their western playground. Keeping them all to double the length of every train near Bristol? Dream on! After spending a fortune on electrics for many GW suburban routes, DaFT's generosity will run out long before then! If they keep ten 'spare' units, they'll be lucky. The rest will be used directly or indirectly to retire pacers.
I'm sure many of all the diesels will find life extension in private ownership, even the pacers. But in their case, not a moment too soon!