If the lenses had smaller apertures. Interestingly allot of 35mm lenses actually have a large enough image circle to accept a DX medium format sensor similar to the Pentax 645D.
The Pentax isn't suitable as a general purpose camera though, and in terms of detail is out performed by a D800E, so it's a hard sell at best.
I'm not saying to buy a 645D thou am I? I'm mearly saying if bigger = better, then why decide on 35mm? It requires to me an almost creationist level leap of faith to assume that something that was convient for film, just so happens, just by dumb luck, to be the perfect sweet size for digital. ********. Utter tosh. The point is there will be a sweet size for digital sensors, and I kinda think it might be smaller than APC-S! I know, burn me for saying this. But its due to how much DoF people actually want, certain manfacturing advances that make smaller censors more sensative and the fact we might actually, finally move away from Bayer patterning in a cost effective way (this will mean no longer favouring green so much, and vastly sharper grayscale).
Digital sensors are a relatively new technology compared to lenses, so that is to be expected somewhat. However I expect most of the inovation to come from the glass itself, rather than coatings etc.
No its mostly the coatings at the moment, the ED technology (or just DOE to use the generic term) of which you speak has been around since the days of telescope manafacturer when they used cast iron canon shots (really good mueseum for this in Venance if your gf is geeky that is) ground against the class, they could create the fisures with grit, but also a quick google (pdf: ) says that it was as far back as the 17th century! Sure we are getting smaller and smaller, cheaper too with improved QC, but ultiamtely glass isn't having any huge steps. There is however one area which is making stella improvements in lens technology.... The lens over the sensor. Each pixel on the sensor has effectively its own lens. The better the lens here, the less prone to detecting the split that creates CA the sensor will be. This is probably the area that will be most noticeable in the next 10 years.
As you say this is in a way due to the relative newcomers of digital imaging sensors. However lets not forget why they where invented. Americans wanting to snoop on Ruskies. Boy howdy did they fund the development of em. It's some 90 years+ old technology, that really advanced during the cold war. It's only in the late 90s we really saw any movement in this area. Let's also not forget that even in 2000 people where saying there was no future in digital, nothing will beat the quality of film, nothing has the fealing of kodachrome. Things change.
I'v been saying this for a while. I think once 35mm becomes upper mainstream, I expect medium format to replace 35mm for most pro's. If this happens, I'll be keeping a close eye on Sony.
I don't see 35mm being around in 50 years, for anything but historians. I just can't see the balance of why its perfect. Medium format was prohibitively expensive back in the day due to reasons that simply don't exist now. When you look at say the 645D vs the D800E, if your in a simple studio environment, the 645D still has the edge on quality, even thou its really old technology. The D800E is much faster and a better all round camera, no doubt. However that does rather show my point about sensor size not being a direct improvement in IQ. This is what I'm trying to say.
I think we will see people go smaller, for economic reasons, for accuracy reasons (its easier with many materials to make them less prone to certain interferences at smaller size). Noise filtering can be improved by certain arrangments of silicon to detect scattering. A lot of this research will be driven by the mid end consumer market, love or hate the iPhone (I'm the latter) but phone cameras are now having more money spent on them, adjusting for inflation, than 5 years ago. There is a hell of a lot of research being done in this area.
So ultimately my point is this, if you've got the few k to spend, and you really care about IQ, then sure, grab your full frame kit, lug the extra weight etc. If your looking to be a 'higher end hobbiest' then its stupid to go for a system just because of FF support. Your lenses are bigger, heavier and more expensive, for a feature you might never need. In the case of OP its simply stunningly stupid to me to go in to debt for a body, which will be obsoleted in 3 years tops, and have a lens that can only be described as "meh" at best. To do that for the reason of future proofing is even more astounding to me.