If mainframe reliability is what it is purported to be, then, for someone making this choice as an alternative to a mainframe, especially if they've been on it

If mainframe reliability is what it is purported to be, then, for someone making this choice as an alternative to a mainframe, especially if they've been on it for decades already, it's a very solid reason. It is, of course, only a proxy for the real reason, which is the engineering culture that led to the bug and/or reliability. If there's better evidence that Microsoft has drastically changed its engineering culture in favor of reliability and/or IBM has done the converse [1], it could easily trump this proxy evidence. [1] for example, if they've included the OS developers in the layoffs they've been in the news for