Given that Raspberry Pi hasn’t actually been well supported by mainline Linux and that the Raspberry Pi foundation hasn’t put a lot of effort into upstreaming things, I don’t think it’s actually a big deal. People don’t care where their kernel comes from as long as it works.
I am surprised by all of the comments here that assume Raspberry Pi has great upstream support. It’s amazing that people just assumed their boards were working great with upstream kernels. Raspberry Pi has a history of doing nonstandard things that serve their community but are actually a little bit quirky when it comes to normal embedded Linux.
I highly doubt so. In fact, save for the RP2040 which isn't Linux capable (0), all their processors aren't for sale anywhere; Broadcom simply won't sell them to you, no matter if you order 1 or 100000. That is, you can't build your product around one of their CPU and you have to put their entire boards in you product instead, which translates in huge costs, no industrial rated parts and forced use of SD cards for system disks, which in that context are a no-no. The RPi is a hobbyist board with a huge potential for teaching, but I wouldn't consider it for anything beyond that use.
0: yeah, I know you can run it in theory; I mean in a usable way.
That forces you to sandwich two boards together (connector, more costs, etc), and still you have no freedom regarding which peripherals to place around the RPi SoC.
The average integrator doesn't want to mess around with things like DDR5 routing or designing power supplies for the SoC. There are lots of other companies such as AMD and Nvidia selling SOMs so the fact that they are selling SOMs can hardly be something to complain about.
This is one of the main advantages for hobbyists.