I think it's obvious that if you own a device - eink or otherwise - that doesn't support the multimedia aspects of ePub3 then you won't buy the books that have that incorporated, just like I don't buy an old vacume tube floor stand black&white tv if I want a portable solution to watch my videos.
Each device has its uses.
However, to stifle technological progress because a particular device doesn't support the new technology is like saying we can't develop horseless carriages because there is nowhere to hook up the horses...or we can't develop microwaves because there is nowhere to stack the firewood...or we can't develop washing machines because the rocks and washer board make too much noise on the spin cycle...
If there is market demand, industry will develop the tools and eventually
the eink readers will be able to support it.
For that matter, as technology improves - memory becomes smaller/faster and you have more of it and processors are smaller/faster - does it really matter that a normal ebook would be 5mb?? It wasn't so long ago that I was the first person to install a hard drive on my PERSONAL computer - it was a whopping 20mb hard drive...I was the king...my how times have changed...I got a 64gb USB drive the other day...
Technology does improve with time.