The system partition is usually the same size regardless of which storage option of the same phone model you get.
But if the system partition could be smaller, other partitions could be larger.
The system partition is made some fixed size, the same way disk partitioning works on PCs, and never resized, because resizing file systems is still a non-trivial task. It often has some free space too to accommodate future system updates.
On my 128 GB Pixel 9 Pro, /data is 109 GB. The rest is /system (although `df -h` doesn't show it explicitly, no idea what's up with that) and various other system-related partitions.
Yes, but if the phone shipped with less bloatware on the system partition, then maybe that partition would be made smaller initially.
Meaning the user would have access to more of the phone’s advertised storage.
If people are paying for upgrades to storage space it's completely reasonable for them to be annoyed by bloatware