Supposedly this oven had a (broken) internal release. It also can’t swing closed on its own and needs to be pushed fairly hard to latch closed. And lastly, there is no way to turn it on from the inside; You need to activate it from outside after latching the door.
It was the combination of all three of these things that had people immediately suspecting foul play. Because there’s no way she could have turned it on while alone; Someone else had to have closed her in there and turned it on, and the broken internal release meant she couldn’t escape.
I’d argue that having them be 30 or 40 years would even be fine. An author who starts in their 20’s should reasonably expect to keep profiting off of their early work until they retire in their 50’s or 60’s. But the current state of copyright is just asinine, because it is basically written by and for corporations.