You don't think treating people differently based on what you actually sold them makes sense?
That's kinda weird.
More seriously though, you seem to be reading the point backward.
It's not that Tesla will "deny" post-March-19 buyers the magic L5 FSD once they have it.
It's that they'll be obligated only to refund the pre-march-19 buyers when they find the sensor suite isn't capable of delivering it.
Because the pre-3/19 buyers will have bought something specific Tesla can't deliver.
The post 3/19 buyers will have received everything promised to them when they bought since that's a much smaller, very specific, list of features.
It really comes down to the fact that you're reading milestones on the order page as the only thing promised, and you're disregarding what's shown on the product page for various models like the Model 3.
I don't agree with this way of looking at it, but I acknowledge that people are doing this.
Tesla shouldn't have the order page the way they do because it's too weak of an offering. It's also a bit of a lie as lots of the features listed don't really work well enough to be considered accomplished. They likely watered it down for accounting reasons, but it's now causing customers to wonder what comes next?
I don't expect the order page to improve either. I hope they add more milestones, but instead I think they'll simply add auto-steer under the list of things complete when they roll out a primitive version of it.
Basically the milestones were simply a way of giving people a sense that something was being worked on, but will go away once it no longer has that value.