First, we were told information about MoS that said it had a 20 year hibernation. If you want to believe that it isn’t accurate since it pretty much an unknown thing, then go right ahead but, unless said otherwise, we should assume this is true at the moment. Like, if Mars writes in that it gets used before the 20 years, then we can say that no one really knows its capabilities. But, for not, we can’t assume that the 20 years isn’t true and just say “Oh, it doesn’t matter if Dragon Monarch and Co kill him bc he can just use MoS and be reborn.”
Second, the idea of an “extra life” as you described it is similar to the Nirvana he did previously where he was “reborn” in his original body with all his original strength/powers. However, this is not what we’ve seen MoS. Like I said before, we don’t know much about it but it doesn’t seem to work this way otherwise when YC was poisoned in the beginning, he wouldn’t have been reincarnated into the YC baby but would’ve just woken up in bed without the whole trip to the Azure Cloud. So, because it does things like “reincarnation” (getting a new life) rather than “reviving” (returning to life), I don’t consider it to be an extra life.
Third, you are right, it IS an Ex Machina. As Elliot mentioned, the MoS also alters karma/destiny so, rather than just being a simple reincarnation tool, it can also be seen as the reason that all the things “happen” to occur that saves YC. How did BB get enough strength to throw a VS to YC when she was being suppressed? MoS. How did YC just randomly appear in the one place that would protect him no matter what (Glazed Light Realm) when it can send him ANY RANDOM PLACE IN THE UNIVERSE? MoS. Hell, how did the devils manage to break through faster resulting in Jasmine to come forward and put herself at risk, resulting in YC becoming the demon god? MoS.
Again, all this is based on the VERY little we know/seen about MoS so maybe this isn’t accurate and it can change in the future. I’m just saying that it gets annoying when people throw up these theories and they haphazardly throw in the MoS and have it be another Nirvana or a time machine when that ISN’T what we’ve seen it do. Its annoying to see people say “Oh, I think YC will go up the DM and attack him, but lose so he’ll use MoS to go back in time and beat him up when he’s a baby, and then die from someone else and than use MoS again and be reborn as True God” since they are using MoS in a way we have NEVER seen it used.