At 300dpi, an A2 page needs about 35MP to fill it, so if you include a white border for the mounting frame, 30-32MP will get you closer to filling an A2 page without needing to scale (if you don't crop.) FWIW, A1 needs >60MP.
The megapixel fights in this thread have some of the most inane comments. If 24MP is enough for pros then why do pros buy 280MP digital backs? The answer is in the nature of the subject matter and work style used. Canon's 1-series cameras are aimed at a very specific set of types of professional photographers but not all. Canon aims for the mass-market of professional photographer with its R1/R3. Mass market includes parapazzi and those filling your trash-mags at the supermarket checkout, or latest online stuff. Mass-market is where the numbers are for relatively healthy unit sales. Mass-market is a relative term here, with digital backs being what I'd call bespoke (for want of a better word.)
At 300dpi a full spread in a magazine needs less than half the 24MP of the R1. Thus the professional selling their pic to news outlets probaly want some slop for crop/rotate but when they're doing high fps to get "the moment", 200MP is of little benefit.
The R6 series of camera clearly isn't targeted at the mass-market pro because it doesn't have all the features they use, or if it did, they'd stop buying R1s and R3s and save some $$.
Why does the next R6 need more MP? I'm going to be simple and say better product differentiation & segmentation between the R6 & R8. The product line will be divided up with the R5 always having the highest MP, the R6 will be next, and the R8 below that. Differentiated in price and MP. I understand that the differences are more significant, but many might not. The R6 moving up in MP may be Canon's response to R6 sales dropping after the introduction of the R8 or otherwise allow Canon to put more features in the R8 to make that more competitive without impacting R6 sales. There may also be some need by Csnon to want more MP than the equivalent Nikon, and blah blah competitive marketing. i.e nothing to do with pros but everything to do with protecting profit.