Here are a couple of examples. If Canon conducted market research asking potential customers if they would prefer that a 70-200mm f2.8 lens can or cannot take a 1.4 extender, it doesn't take a rocket scientist to figure out most customers would have picked the ability to take an extender.
Virtually all RF lenses have pushed new features over their EF counterparts. Only Canon would know how much the new RF mount contributed to easier optics for a compact lens but the RF70-200mm/2.8 out performs the EF version in all areas except for the ability to handle extenders.
It is a reasonable commercial decision for Canon to gently direct RF users to get a RF100-500mm and worth a few customer grumbles.
I had the ef70-200mm/2.8 and both EF extenders and didn't buy the EF100-400mm as the set was cheaper and smaller but was slower and image quality @ 400mm wasn't paramount. The lack of extenders with the RF70-200mm pushed me to the RF100-500mm and I haven't looked back
Canon still allows you to user adapted EF lenses so there is no downside - right?
Similarly, if Canon polled customers and asked them if they wanted a 100-500 mm lens to have a maximum aperture of f5.6 or f7.1 at the long end, or that such a lens would take an extender through its full range or only from 300mm onward, it's safe to assume most customers would choose the faster lens and the wider zoom range.
But, market research cannot overcome design limitations and the decisions that limited the features of these lenses certainly came down to design issues, balanced against an informed decision about what the downsides might be for marketing the lenses.
Customers would always welcome new features and Canon could provide all of them at a cost. The old engineering conundrum... size, cost, features => pick 2
If Canon asked... we already have the EF100-400mm that can take extenders and can be adapted to to R mount. What different features and constraints would you want to see and accept in an RF version?
Lower weight and size = tick
Effectively an integrated 1.4x extender by increasing the focal range to 100-500mm = tick
Same focus speed and minimum focus distance = accept
Same front element/filter ring size but slower than f5.6 @ 500mm which is practically the same as EF version + 1.4x to keep cost/weight from being too high = accept
Compromise that to get past 500mm that you are constrained by the 1.4/2x extenders to start from 300mm = accept
New cost is higher than EF version = accept or use current EF version.
I recall a Simpsons episode where Homer gets to design a car with everything he wants in it but bankrupts the company in the process.
There is also the great unknown -- the rumored R1. No one on this forum has any idea what the features of the R1 will be (or even if there will be an R1). But, we can all assume that the R1, if it materializes, will be "better" than the R3. We just don't know what "better" means, even though everyone has an opinion on that.
All this is a long winded way of saying it might be prudent to dial down the insults a bit since we all are posting from a position of ignorance.
Our position of ignorance is correct but we all have opinions that hopefully we can logically justify our own stance. That said, wish lists are fun.
Insults aren't cool but making making generalisations assuming that they have a coveted position of knowledge.
We would love to have a contributor with that knowledge commentating!!
Hopefully, participants in Canon Rumors can learn from each other as I certainly have by others challenging my posts