Lol as someone that owned 4 Zeds including 350Z a few owners got rust protective treatment which includes undersealing at 7 to 10 years. I think you're exaggerating the case of the F Type. Subframe issue is just a case of removing, treating, resealing. Subframe can be had for £700...I cant speak of a lack of parts for an older jaguar but I still see a lot of old jaguars on the road so maybe the owners are finding parts. I know with Nissan parts are still made by Nissan and aftermarket companies for the older models
In 2008, JLR was purchased by Tata from Ford. They had a range that was entirely Ford developed. Pre-Ford cars are well served by the aftermarket. For the X300/308, all Ford did was make existing Jaguar designs better. The X100 coupe, underneath, has a lot of XJS there.
It changed from the X200 (s-Type) onwards. These were all Ford developed cars. In 2008, all of this became Tata owned. FoMoCo maintained supply agreements for a few years. For a car like the Jaguar X350 (the aluminium XJ), it was only made for 6 years, and wasn't a big seller. Tata seems to have shifted some of the supply chain to China. Ford is no longer interested in supplying parts for an old car from a brand it no longer has an interest in. The major driver for the Ingenium engine range was Ford ending the supply of Duratechs.
Based on JLR's own statements, the future of the Jaguar brand is very doubtful. The X150s, F-Types will become orphan cars quite rapidly.
Jaguar cannot be compared to Nissan. Nissans sell in huge numbers.
My 2004 Jag was off the road for 3 months, while the last V8 thermostat housing was tracked down. Prior to that, I needed a drivers switch pack. I have the folding mirror model. Switch packs are only available for non-folding models. A £500 new part had to be cannibalised. I needed some new rear calipers. NLA. On the way home from a funeral, I head a crunching noise. A week after a MOT, one of the tank straps had let go. NLA from Jaguar.
Jags aren't particularly reliable cars. They are typically £70k cars with horrific depreciation, but still with a £70k car maintenance bill. Given the change of ownership, which seems to have lead to the normal stockpiles of obsolete parts being disposed of, and the treatment of the supply chain by Tata, I cannot recommend a Jaguar from the modern era.