In my copy of
The Dancing Chain there is a period drawing of an early 1930s bike with a rear derailleur only (rear deraileurs were fairly rare in the early 1930s, front derailleurs were extremely rare), a single chainring and a
small chainguide to keep the chain reliably on the chainwheel.
They learned. Cyclists since have learned over and over and over and over...
That Tern Mainstay Chain Guide is petty slick an very reasonable.