From 1981-1988, I ran the the Market Data II system at at NYSE.
I converted it from a IBM /360-50 with 2703 Frontend (modified hardware) to IBM 4341 with 3705 Frontend (unmodified hardware).
Software in mainframe and Frontend was modified from IBM software.
Backup system was another 4341 running IBM’s VM, normally used for test/development work.
During those 7 years, we had no software failures.
We had one hardware failure: fullblown red-light-sparks-and-smoke crash.
We were back up before the operators could get to the fire extinguishers.
Total downtime was about 47 seconds.
Figuring 8 hours/day, 5 days/week for 7 years: that’s uptime of 99.99987+%
Modern mainframes are much more stable than that 4341. (Mean-time-to-failure about 25-30 years).
Anyone really think the Intel-based world will every match that level of stability?