" saying "they have the VHDL, a chip can be produced at any time..." is a bit naive."
except... I didn't say that.
I wonder what controls how long past the last time buy a chip remains available in the "cowboy" market? I have seen about 10 years. I bought tens of thousands of the H8/323 at $6 when the official price was GBP 9, which was surprising since the cowboy outlets normally sell at a premium.
Another was the Atmel 90S1200. We last paid GBP 0.50 and I would buy a few k again if I saw them. Atmel replaced that one with another chip but by the time I got around to changing the design they discontinued that one too I will need to revisit that sometime... or find some more 90S1200.
The last remains in UK:
https://www.silicon-ark.co.uk/at90s1200-12pc-cmos-8-bit-microcontroller-by-atmelI think the "after-EOL" of a chip depends if it was used in some "millitary strategic project", where they've stoked large amounts and after some years, when the devices using them are fully retired they sell the stock to brokers.
Same with large corporations, that when finally the product they were compelled to support is gone, they're clearing the stocks and of course, the out-of-business situation. But this really does not have any kind of predictability and one can't base his business model on it. It's more of a desperate measure, like when NASA was asking for ceramic 486-25 for some radiation resistant computation module or such and some "cowboys" made bank.
In the end, either your product is important enough and have margins to assure you have enough parts for the whole life-cycle, or not, and then have to deal with the possibility of your original parts drying up and the need to redesign. For the consumer stuff that's no big deal, it may even come cheaper and more features can be added, but for projects where there is a significant certification and/or regulatory burden, one can be severely bitten by neglecting the supply chain, "cowboys" are not always available.
The auto industry (and not only) tried to apply the "new supply chain paradigms", with "elastic stocks" and "just in time ordering" and they're kicking themselves over it, the loses incurred because they didn't want to stock semiconductors and other parts are now way bigger than just buying the stuff and keep it in the storage. So many other in the industry that were doing TED talks with a superior smile saying how cool they are that they've moved the burden of stock to the manufacturers/distributors and how they order only what and when is needed. This model only works with a perfect running supply chain and even a small disruption will crash whole industries.