Some things would need to change to make this statement true. Stock brokers and portfolio managers would have to be forced to stop reporting gains without basis. Without the basis the IRS has no idea what your gains were.
Fidelity reports the basis in a supplement section in the same damn document. Moving them together would be trivial, but I've been told the law is forcing them to do it this way (they didn't tell me that, I just read that somewhere. Not sure if it's true).
As someone said, almost no one has investments. If you remove 401k, it's a tiny fraction of people. If the only thing I'd have to do to file my taxes would be going online, finding the list of my sales and needing to add the cost basis then click submit, it would still be a huge improvement compared to what we have now.
Still, it's a problem that's pretty easily solved.
So, yes, such people are already using the simplest possible form, but ease of filing is a big deal, and I suspect many people are like me and use online filing software for that just as much as ease of filling out the forms.
I also only just realized that the IRS no longer just mails everybody a form 1040 and instruction book.
It isn't like that though. The IRS has nearly all the pieces it needs, if not all the pieces, to create a system like the UK, where they just send you your return and you sign it unless you think there's a mistake. But that system doesn't exist because of companies like Intuit who lobby hard to prevent it from existing.
In essence, Turbotax is solving a problem that Intuit is creating (or at least lobby to prevent anyone from fixing it).