I'm dismayed by Elon's comments about stamping dies as critical path. I can't imagine why. The sheet metal design and form has been locked for a long time.
Let me try and recall from my memory why there is such a long lead time for this and why it's not like slapping a bunch of Legos together.
First you design the car and break down its individual parts.
Then you have to design the die sets to make each part (yes, SETS as in each part will require 4-5 individual dies to make the part).
There are die standards that have to be followed and are extensive.
There is a slew of information that is required that the design has to fall within like; what is the exact material (type, thickness, supplier) that will be used, what pressline will be used and what are those specifics (size, tonnage, stroke rate, shut height, cushion pressure range, clearance for robots, scrap chutes, stackers etc...).
Once all of that is taken into account then you have to run computer simulations. In the old days there was no such thing as computer simulations, so there is some chance to cut out valuable time here and down the road via mitigation of possible problems. However, there is such a thing as bad die design, in which case no amount of changing the die design will fix it and you then have to go back to the drawing board.
During this whole process there will be die design reviews FOR EVERY SINGLE DIE (we can be talking upwards of 200), checking the progress of the design, making sure something hasn't been overlooked, something isn't missing, a mistake hasn't been made, the die standards are being followed etc...
The next step is then to order the castings. I can't remember the average turn around time for castings, but it's got to be a few weeks from start to finish for one die - including shipping time.
Then you get to start making the blocks for the die. In the old days that was all done by hand. Machines and computers makes that process a bit faster, but not a lot because you still need someone to program the machines - and if they get it wrong, you get to start all over again.
So you make all the blocks, punches and assorted bits and pieces and 'build the die'.
During this time there will be die reviews making sure die standards are being followed, that progress is on time, blah, blah, blah.
Then you get to take it apart and send all the pieces out to be heat treated. That mostly goes okay, but sometimes it doesn't. Sometimes a block will split, get damaged, get 'misplaced' and you get to make it all over again.
You get all your blocks and parts back and you get to put the die back together again. Then you get to put it in a try out press and see if it works. I can tell you, you never get a part in spec at this stage because the computer simulation is never 100%, because mistakes get made, because sometimes the material supplier goes out of business or no longer makes that material or changes their formula, or any number of other things that can and often do go wrong.
So at this stage you get to make changes to the die. Sometimes it's a change to a cutting edge, sometimes it's a different punch, sometimes it's grinding down a block or welding a block up and reshaping it. Sometimes it's making slight changes to the press, increasing or decreasing cushion pressure, slowing down the stroke rate, increasing or decreasing tonnage, blah, blah, blah. Hopefully it's a quick and easy fix - lots of times it's not. If the part is simple, sweet. If the part is complicated, not sweet.
And then when you've produced a part that's 'mostly' within the company's specs (there's a percentage range that companies will accept, (ie. might be 80% of the marked points have to be in spec) they show up to do yet another die review. If all the criteria is met, you ship it (takes time) to them and then they get to go through a whole set of homeline trials (typically 3 stages), which can take weeks to get through to produce a part good enough to make a car with. Why weeks? Because the dies were most assuredly NOT tested on the exact same press, using the exact same material (as in from the same supplier) as when they end up on at the company. So you don't just get to slap them in and voila they work. Tesla may be able to mitigate some of this because they own the tool and die company, but I can guarantee that one company is not big enough to be making all the Model 3 die sets - certainly not in the time frame.
I've probably missed some steps in there, but that should give you the gist. It's a long, tedious, complicated process. And there's a whole other level of exactness required to produce a die set that makes a Class A body panel, than a die set that makes TVs or barbecues.