As a non mainframe programmer I found your anecdote about windows reliability contrary to the experience I have had over the past few decades so I am not sure how accurate the rest of your post is.
article about it: https://blog.ctm-it.com/it-support/blogs/matt-cannon/2013/49...
edit: If you want to be especially cynical, you can note that this wasn't discovered for a while because everyone running Windows reboots it regularly because of its notorious instability issues (the bug was found in 2013 and dates back to at least Vista, which was last updated in 2009).
I reboot monthly because getting owned by 0 days sucks, and at this point in life, every platform (except for OpenBSD) is having new exploits found against it at a rather fair rate. Same reason my phone gets rebooted once a month.
Aside from that, I have had uptime on Windows boxes in excess of 6 months.
Laptops on the other hand, those are more problematic, but it isn't Windows' fault if a WiFi card hard locks itself, has a dysfunctional watchdog timer on board, and needs to be power cycled to get it working again.
OSes that can hot swap or live patch kernels can brag about uptimes. The rest of us should stay humble. (And AFAIK no modern consumer based OSes distributions do that)
Windows laptops.
My MacBook has been up 56 days 14:06 hours.
Apple laptops have their fair share of hardware issues. Windows laptops tend to have more due to a shorter product cycle. Apple has longer relationships with their parts vendors and thus they have more time and leverage to force good firmware onto the multitudes of independent chips that make up a modern day computer.
Windows used to suck, but it's somewhat unfair to judge the platform of 2018 because Vista of 2006 was bad for you.
Hopefully a mainframe expert can chime in as to whether the major mainframe OSes have included similar or worse bugs.
Windows has improved since then, but it's still a very brave person that would have a single business-critical Windows machine without a few scheduled maintenance and reboot periods every year.
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