[Klaus]

It is generally a misconception to think that a 24-70mm f/2.8 (also 16-35mm f/2.8 or 70-200mm f/2.8) are better because they are more expensive.
They are primarily more expensive because they are faster - meaning that the manufacturers have to put in extra efforts to maintain high quality at faster speeds.
Optical challenges increase exponentially the faster you go. That's a primary reason why the price roughly doubles per extra stop (and they like the extra profit margins from milking pros).

Of course, that's just one aspect. Whether a design is excellent or fucked up also depends on the lens designer. e.g. the EF 24-105mm f/4 L was a dud on high mp DSLRs - but that wasn't because it's a 24-105.

In this specific case, I don't see that the RF 24-70 IS is worth the extra costs if it's only quality you are after and you don't care about f/2.8.
The RF 28-70mm f/2 is better than both, though.

Overall, I'm not a huge fan of 24-70mm f/2.8s. They are relatively fast, yes, but it still a far cry from any prime lens. Even a measly 50mm f/1.8 is way faster at a fraction of the cost.

just my 2c