I think the primary issue is that it's poor business practice and it lets fans and investors down. Think of the stock market. When a company releases expected revenues and they are conservative of what those numbers will be, when they over-deliver, people invest. When a company consistently is conservative with their numbers and consistently over-delivers, that stock continues to climb. When a company however, over-estimates and under-delivers, the stock values drop.
Elon could have gone conservative with the specs 4 years ago and still have been offering a one of a kind, nobody had one product. At the time, I don't think anyone yet, had released an electric truck. Then when these specs were finally released now, if they met or exceeded the expectations, people would be happy and excited. Then, in a year from now, as you are pointing out, if they push the limits and jump into the 500mile range, they look even better.
Video games people looked forward to for years end up failing because on release date, all of the things they promised, haven't been implemented into the game. By the time the developers finally get all of the features into the game, the hype is over and nobody is playing it anymore.
Tesla created a great product and has had a great run, but Elon is still banking on the fan boys to keep him above ground. The hype is not what it once was. It's like fans being disappointed with the movie The Matrix 2+. Because the most important aspect that made the first movie so awesome "What is the Matrix?" was answered in the first one. There was no longer that mind blowing concept. Electric cars are here. It's no longer a mind blowing concept. Other companies are building them and selling them. Tesla needs to back down on the ego and focus on the facts. "We can make this size battery pack put out 300 mile range currently. We plan to put the same physical size pack in the new vehicle that will be releasing in 4 years. We can estimate that the battery tech will have improved so much that we will get 500 mile range out of the same size pack, but lets stay conservative with 300 miles and then if we hit 500 miles, it will be a huge surprise to our customer base and excite them" is how this should have been approached.
In the end, they are using old tactics that worked because of all of the hype. The hype is dying. They need to change it up and start being conservative with specs of future products or they are going to suffer in the stock market dramatically and Elon is going to have to find someone who wants to buy X and turn it back to Twitter to keep Tesla afloat