Cartridges and hardware don't last forever. Even stored properly in the original box, my first ColecoVision was dead within 8 years because of board rot. Don't confuse longevity with availability -- hundreds of millions of cartridges and consoles have been sold in the last 30 years, so there are bound to be plenty of survivors. How many ColecoVisions and other old hardware died for every one that is still working now? I suspect it is higher that 10:1.
My experience with older NIB cartridges is that about 1/25 are DOA. None of the older NIB optical games I've purchased have been DOA.
I don't find optical media to be fragile, but I handle the disks by the center and edges. I have CDs that are over 25 years old, whereas the cassettes and floppy disks I had from the 80s are all degraded to be virtually useless.
Even in the early days of optical media, I had a hard time understanding what all the whining was about with loading times. A few seconds wait time between levels does not ruin my day. There are enough real world things for me to get upset about if I need something to piss me off.
That said, some developers are notorious for making games with extensive load times. EA comes to mind, regardless of the console generation or hardware. The PSP is a great example, with many complex games having short load times while others are ridiculous. If a puzzle game or arcade compilation has a 15-30 second load screen there is something wrong with the way the developer setup the disk. Currently most developer-publisher relationships demand the game get out the door quickly to recoup investment, often before the developer feels it is ready, which too often leaves a game unoptimized when it reaches the consumer. These "almost finished" games have longer loading times and more overall glitches with presentation and mechanics. You know what I'm talking about, I'm sure you've played at least a few of them over the last decade.
I agree that we're going to see the end of both cartridge and optical media for game delivery. The migration has started with the trend of "network-only" releases and will only escalate. As cheap as optical media is to produce, network delivery is even cheaper. Even better for the publishers, the end user will not have a permanent copy to go back and play years later. "Buy" a new download or "rent" it if you want to play it again. It's shaping up to be a grim future for collectors and nostalgia gamers.