That limitation was not about frequency, it was about supporting widespread TV hardware. Flickering was inherent to how TVs worked at the time, but Amiga software did not sufficiently account for that. If they had used the interlaced modes correctly, i.e. as a minor tweak over simple scanline doubling rather than a true increase in vertical resolution, the flickering would've been a non-issue.
The Amiga only displays one interlace field per 1/30th of a second -- that is to say that it is effectively 15(ish)fps in interlaced mode. It's only using a traditional 200 line output, just phase shifted for the second field. It's basically a hack.
It flickers because the persistence of most CRT screens is less than 1/15th of a second, and so by the time you see the second field, the first field is already diminishing in brightness. IIRC Commodore released (or wanted to release?) a monitor that had a longer persistence.
> The Amiga only displays one interlace field per 1/60th of a second -- that is to say that it is effectively 30fps in interlaced mode. It's only using a traditional 200 line output, just phase shifted for the second field. It's basically a hack.
That's how TV's worked back in the day, yes. It would have led to flicker when you had lots of sharp changes in image content across scanlines, but not otherwise. E.g. filling the whole screen with a solid color, or even a smooth color gradient, would've worked quite fine in interlace modes.
Unfortunately, this inherent limitation of interlaced display went unrecognized in the Amiga dev community - everyone would've known that interlaced screens tended to flicker a lot but most devs didn't know why, or how to properly work around the issue.
That wasn't my experience at all. Everyone knew what caused flicker and how to work around it. For example, that's why Workbench 2.0 had a much lower contrast black on grey default color scheme, instead of Workbench 1's more garish white on blue. (Supposedly those were picked to look more readable on cheap monitors.)
The problem with running Workbench in interlaced mode is that it just gave you tiny text and icons: the ROM only had a single bitmap font in 8 and 9 pixel height size, and adding custom fonts to your boot disk would've taken up a whole lot of space.
(Similar to running unscaled High-DPI today on systems that weren't designed for it out of the box.)
