It's not about seriousness or anything like it, the issue is any kind of tonal and narrative consistency is gone. Imagine if Dog ending in Silent Hill 2 was the only and canon one. 95% of the game you are building up towards one thing, focusing on one thing, building up towards one thing, then you do ridiculous 180, abandon everything you built up before and go into a new direction. This is just terrible writing 101. Suddenly making things that were never relevant for 99% of this already incredibly lenghty game a focal point, while severely downplaying other issues.
Similar things happened with Reborn, it was absolutely fine going into final versions, but suddenly they realised they are at the champion and they barely started their plot because they never had coherent vision. The result is the asinine postgame that completely ruined the rest of Reborn. Any work of art need a coherent vision and tone.