Sort of disagree. Unless you are talking about a $2000+ full-frame DSLR, only a small portion of the camera is the sensor. The vast majority of the image quality is coming from (outside of the operator) the optics. Unfortunately, good optics are hard to minaturize. There is a reason why my wide angle zoom with a constant F2.8 has an 82mm diameter filter, whereas you could get a cheap one with a variable zoom (like a f4.5-5.6) with a filter in the 55-58mm diameter range.
As for the high quality mirrorless camera's I wonder how much light is lost since I believe they use semi-transparent mirrors, which only half the light goes to the sensors. Since I like taking pictures before sunrise, after sunset and when there isn't much light, I want my sensor getting all the light it can get.
They may be getting better, but in the mountains, I am like Sierra LedgeRat (like that handle man), but my dSLR is my main go to camera.
Actually, the mirror in a dSLR gets in the way of optical quality. Without a mirror, the lens can be designed such that the rear lens element is closer to the focal plane. This allows the lens to be made smaller, while preserving (or even increasing) optical quality. With an SLR or DSLR, the distance from the rear element to the focal plane partially dictates the large size of the lens. Get rid of the mirror, move the lens closer to the focal plane, and the lens gets smaller, and optical quality increases. The very best lenses in the world are the Leica M-mount lenses. The optics on these lenses are much more compact than dSLR lenses. They are smaller in part because they were designed to fit a mirrorless (rangefinder) camera.
It's true that it has traditionally been easier to find high-quality lenses for dSLR bodies than for compact cameras. The reason that lenses for dSLR systems typically are better than for compact systems isn't because good optics are hard to miniaturize. It's because of history. dSLR lenses are better because up until very recently, dSLR bodies were the only option for serious/professional photographers (with the exception of the Leica M system.) Compact cameras were targeted toward beginners or non-serious amateurs. This is changing rapidly now. As the compact camera segment matures, smaller, high quality lenses are becoming available (in addition to the Leica lenses) that are every bit the equal of bigger lenses for dSLR systems. A good example of this trend are the Olympus 12mm, Panasonic 20mm, and Olympus 45mm for the micro 4/3 system, and the Zeiss line-up for the Sony NEX system. I'm shooting with some of these Olympus lenses, and they are comparable to the Canon "L" lenses I used for many years with my SLR system. The new Fujinon Pro line will likely also be very good.
You can get high quality lenses and pro-level features in a compact system today, if you are willing to pay for it and if you do a bit of research on what is available. Going forward, it's only going to become easier, as more and more high-end optics and bodies become available in the compact segment.
As for your concerns about low light performance, I think that real world results from cameras like the Fuji X100 and NEX-7 show that compact cameras can have low light performance that is equal to (or better than) that of dSLR's.
A good summary here: http://www.stuckincustoms.com/2012/01/0 ... he-future/