i think that what motivates a car co, like any business, is money; as in- development costs vs return. rarely do you have a stud like piech who says- we are gong to make a bugatti even if we don't make much money or any money at all. as such, i don't think it is as much a matter of laziness as it is a matter of timing. audi saw the RS4 engine as a cheap, easy to install engine in the RS5. perhaps they were already on the road to developing the next HO V8 wiith 500 hp but it just wasn't ready- and so- rather than delay the RS5 or not have one at all they decided to package it thus. i'm sure lexus fanboys hate the ISF engine- it is the last "relic" V8 to go into a that kind of car, vis a vis the gas guzzling peers that BMW and Audi and even mercedes makes.
there is one thing to be said though. what can differentiate one car company from another is its ability to foresee trends and plan develoment timelines efficiently. audi will need to step it up and hopefully it has developed a stout, solid, flexible new V8 that will be what it needs to compete for the next ten years, with the built-in flexibility to make the engine insertable/modular into multiple applications. for example- i hope audi has had the foresight to see that this engine will be essential for the powering of the next A6 all through the next R8, with an inherent production process that will not delay the production of any of the cars it is going to go in yet still be malleable enough to be modified to meet potentially changing needs (such as a hybrid system). the same should be said of the upcoming rumored V6 engine AND the current I-5 that is supposed to go into the next midrange of sportscars in the line up.