I'm not sure what was the use case back then, but today plus-sized 4k+ screens or ultra wide screens are commonplace. There's simply too much real estate for a single application window.
It's not a binary choice between tiling applications and just one at a time. Overlapping windows are a thing, and arguably were the thing in a lot of windowing systems for approaching four decades now.
I did not make myself clear, yes it is not a binary choice. Yes overlapping is one of the solution. Hand-managed layout (where you just place a window wherever you want) is one too. And so is tiling, which counters the point the original comment made (no use case for tiling WM in modern era).