That is practically unheard of in the game industry.
Another thing that was unheard of in the industry...a pre-order bonus (Nevada) being 4 years late after the product released...lol
And that is relevant in what way?
I'll try to explain how it's relevant.:
You say that developers giving free upgrades for years is unheard of...this is true to some extent...but no developer in history has screwed over their loyal customers as much as ED for the last decade...period. Your BF1-BF5 comparison is not even close...those titles were released in a reasonable state and updated in a business like time frame at the time money changed hands. So I was thinking that it could be a good opportunity for ED to flip the script. Possibly give the customers that purchased BS1& BS2 the BS3 update for free.
I can hear you saying already "You wouldn't work for free". I don't charge customers full price and then not deliver a finished product...it seems software development is one of the few exceptions that allows this type of fraud. They could shock the flight sim industry by actually throwing some sort of free nugget to their loyal customers but I think we know better than that.
That still isn't very relevant, and not historically true with regards to Battlefield 4. It took over a year and had to get handed down to a different studio (DICE Los Angeles, former Danger Close studio) for the fixes to be properly implemented. While that doesn't mean every game should follow this example it does show that it isn't that uncommon. Even the recently released CoD Modern Warfare was broken for about a month post release because the tick rate / net code couldn't account for the rate of fire of weapons. This resulted in instant "death rays" because only the first shot would report on your PC as a 3-5 round burst would be combined into one shot. So turn around a corner, see an elbow sticking out, boom, instant death. They fixed it and now you can see enemy shots, every one of them, and feel the gradual impact of them. But this is coming from the series that probably made the most profits this past decade. That just shows you how common it is for a full price game to include many bugs.
As for ED giving free stuff, we do get updates. New graphics, AI units, functionality, a new free map (thankfully) and an entirely new 3D model for the Blackshark cockpit. Only thing you'll have to pay for is the new variants. While the A-10C represented a current model for the time, it was quickly made out dated due to new software suites. I recall reading a few years back from someone who worked with the planes that the A-10C was fairly close to the real thing but was a very out dated suite. Consider it something like making ah AH-64D circa 2000 flight simulator in 2008, and then expecting a free upgrade with all the new technology in the AH-64E a decade later.
Fixing things and bug fixes do need to be free, and the upgrades to the core game are needed because it is lacking in many areas. But I can see why they are charging a bit for the new models of these planes. It depends on how much of a discount they give. If they want ~$30 for a slightly newer model then it certainly isn't worth it unless you absolutely need the latest model. But from my understanding you'll still be able to fly the old variants if you don't purchase the new ones.
Pre-order bonuses being late (very late) doesn't have much to do with what products should get free updates for X amount of years though. I would say, if ED can figure out who pre-ordered the A-10C way back when, those people should get the new variant for free due to having to wait obscenely long. It would certainly be helpful from a PR perspective.