I'm not going to wait two years -- though I'm opposed to preordering -- but there are other benefits too. Two years down the line:
A bunch of bugs are patched. Even if Starfield is relatively free of bugs, there will be some.
The wikis for the game have been written up. Some obsessive person will have sat down and figured out the quirks of game mechanics and documented them. Understanding stuff like the relative merits of armor-piercing, bleeding, and so forth in Fallout 4 was complicated.
Starfield's expansion packs will be out.
Mods will be out, and there will probably be some pretty "must have" ones.
You'll have more hardware oomph to throw at the game, make it smoother/higher res.