It feels to me that ram increases in iPads are pegged more to the features Apple wants to enable, than any specific timeline.I don't think it's an impossibility.
The iPads had:
1GB 2012-2013
2GB 2014
4GB 2015-2018
There's 50/50 chance we get 8GB on the new Pros. If the A14X/Z are close enough to M1 (or perhaps are M1s that didn't pass binning for Mac-level), it may be more expedient or cheaper for Apple to just go 8GB on all of them.
For example, the iPad Air 2 got 2gb of ram, which I assume is the minimum necessary to support split-screen multitasking (which was probably delayed to 2015 because the software just wasn't ready).
The iPad Pros got 4gb of ram because that's what Apple feels is necessary to support meatier workflows like video editing, more aggressive multitasking needs and the higher resolution of the 12.9" iPad Pro.
The 2020 iPad Pro got 6gb ram presumably to support the lidar camera and possibly some future AR feature, same with the iPhone 12 Pro and pro max later this year.
Which raises the question - what could the next iPad Pro refresh get that might justify or require 8 gb of ram?