Besides people were doing long exposure since like forever with great success. If the sensor overheating was a problem, it would make sense to introduce a hard limit and some cameras have them, but from my experience the likely reasons for the prevalent 30s are: 1/ most people don't do several minute exposures 2/ they need to sell remote triggers to somebody.
30s limit was there even during nineties when you could do hours on film without overheating anything.
One thing that most people comparing a film camera and digtal camera by exposure lengths is the fact that a digital cameras image decreases the longer the chip is energized. Heat builds up, and the image gets more noisy the longer the exposure. One rule of thumb for time lapse is to have an interval in between shots as long as the exposure being taken to allow time for the sensor to cool back down. YMMV
I often need it for HDRs at night. I know I can do like wayloads of high iso noisy short exposures and average them in post, but it's clunky and even harder to set up automatically. I would prefer something user friendly. When I'm shooting at night, it's often cold and even not so safe. Fiddling with apps and cable releases is not something I'd like to do.
I know that the hardware is capable of it, but they just have to sell you that cable release or what not in order to unlock the capability, which is what I'm not happy about.
You don't have to do high ISO anything. If you're willing to shoot 1 hour exposure (which are you really doing that on DSLR?), seems like you'd be willing to shoot 30 2-minute exposures, or 60 1-minutes. What doesn't motion blur out in 1-minute shots other than maybe cars at a red light? Also, how many 1 hour exposure shots can you get out of a single battery charge (2 batts if using grip). I know doing 45s exposures with short intervals will chew up batteries for me.
It seems to me that Canon has decided that if you're going to do longer than 30s exposure, you're going to be using bulb mode and an external controller. I know MagicLantern allows setting custom bulb exposures, but I haven't messed with that option. Its built in intervelometer isn't reliable for short intervals (anything 5s or less is iffy regarding consistent shooting frequency). With my wired controller, I can do 1s gaps. Anything less, and I start getting issues with dumping to card reliably on the old body. Using my custom arduino controller for .5s gaps with 1s exposure with camera mounted to car for motion blurry goodness freaked the mkII out for some reason. newer bodies handle it much better.
Second, an hour was just an example. Most of my exposures are not longer than 4 or 8 minutes.
Third, I'm happy that you are fine with external controllers. But I'm not. I find them crude and unnecessary. I'm doing a lot of HDRs and I don't understand why I cannot simply preprogram my camera with sequence of shots and execute it without any gadgets and limitations. Say, there's this nonsensical limit for 1 minute or 30 seconds everyone seems to defend for some strange reason, (is there some sort of Stockholm syndrome at work?), at least the manufacturer could provide a better bracketing setting to compensate for it. It's not like it's rocket science. An intern could write the firmware in a few days. I cannot even patch the camera without voiding the warranty. I know they are always workarounds, but why make things complicated, when they could be simple.
The mkII got video basically because someone realized the chip could do it, and stuck his head in an office saying they could make it happen, and then did it. It pissed off the pro-video camera guys to no end. So, I'm guessing Canon is not going to let that happen again by letting the interns loose. It's goes against Sony's DNA to let features go out for free.
But seeing as this is HN, many a device has been launched of people tired of status quo and went and did their own thing to make what they wanted. You're welcome to post us a Show HN when you've fixed your problems that will make things easier for the rest of the world too. Or, better yet, sounds like it's ripe for disruption, so why not get a YC backed startup going to build a better camera. Try not to make the camera equivalent of Homer's car though.