I use my iPad for ‘serious work’ whatever that is. I have a 32gb i7 Mac mini and for a lot of stuff I do my 2018 iPad Pro 12.9 runs rings around it.If the iPad allowed true multitasking then I would agree with you. Now, it is worth the extra money to get the 16GB model? For me definitely no since I don‘t use the iPad as my main device. I have an iMac with 48GB of RAM and a 16“ MacBook Pro with 16GB of RAM for serious work. If Apple gave us the option to add 16GB to all storage models, then I would of course get the 16GB option.
I also think that iPadOS in its current state is perfect with 8GB of RAM.
iPadOS isn’t the only thing that requires ram, in fact macOS doesn’t even *need* 16gb of ram to work. Or even 8. The apps need it more so than the os no matter what system you use.