Why? What would a backdoor in the TPM do?
Another implementation could be used to tie DRM to hardware and then brick any hardware that is in license violation. Actually I can think of many scenarios that have a unique identifier on the hardware could be used/abused by corporations.
It should be noted that I lived through a time when corporations wanted this same setup on televisions. Each TV would have a unique chip that would give corporations control over what a person could watch, or even brick their TV. This was called the V-Chip. [0] And then of course, there was also the Clipper-Chip [1]. In terms of boiling frogs [2], TPM could be the base framework to slowly reintroduce the Clipper-Chip under a different name. i.e. used to boot-strap the pieces of code required to watch movies, buy things, browse the web, anything you can imagine.
[0] - https://www.popularmechanics.com/culture/tv/a19408909/20-yea...
You have to trust every manufacturer of every other chip the same way also, and we have in fact found incredible breaches of that trust already many times over, and so it's too late to suggest it's a crazy thing to worry about.
The only saving grace with other chips is thst they are generic and at least you usually have your choice of some range of suppliers.
Will I be able to buy a laptop with an intentionally defective or fake tpm if I want, which will allow me to use modern software without trusting anyone?
Probably not. Probably plenty of Chinese manufacturers would be willing to produce them, like all the HDCP defeating hdmi switches, but probably software vendors will have some way to detect and invalidate them.