I don't think that many manufacturers outside Germany bother to supply anything beyond their minimum obligation, which is to supply maintenance parts for 10 years after the model is discontinued.
I play with old Jaguars, Lotus and Fords, and in all cases most parts are available from after-market suppliers, with Lotus and Jaguar particularly well supported. To restore old Fords, like Lotus Cortinas, Escort Mexicos etc.,you have to rely on a patchy network of suppliers for bits and pieces, and ebay for new-old-stock and good 2nd hand parts.
Panels are crazy prices, with a bodyshell restoration often using £10k - £15k of reproduction panels. More if NOS can be found. But it's the chrome and stainless steel trim that sells for truly eye-watering prices! So when you see a Lotus Cortina or an Escort with BDA being sold for £80k or £100k, it cost all of that to restore!
I guess the simple answer to you question is that the large mass-production companies can't see beyond unit cost and unit profit, and probably that last thing they want to do is support your efforts to keep one of their old cars on the road beyond its 10 or 12 year life. Heritage is something the German companies seem to be proud of, but for most others is just a term to use in the marketing department.