All my SaaS products run on a Windows server, with SQL Server as a database and ASP.NET on IIS running the public sites. You can probably come up with a lot of uncharitable things to say about those technologies, but "flimsy" and "fragile" likely aren't in the list.
As a result, when things go seriously wrong, the application pool will recycle itself and the site will spring back to life a few seconds later. Actual "downtime", of the sort that I learn about before it has fixed itself might happen maybe once ever couple years. At least, I seem to remember it having happened at least once or twice in the last 15 years of running this way.
There's a Staging box in the cage, spun up and ready to go at a moment's notice, in case that ever changes. But thus far it has led a very lonely life.