the ideal font size for main body text is supposedly between 50-75 characters per line [...] this is a very common rule of thumb

There are lots of common “rules of thumb” in typography and graphic design. Unfortunately, a disturbing number of them turn out to be little better than old wives’ tales, with precious little scientific evidence to support them even as well-meaning people repeat them as some sort of objective, absolute standard. As far as I can tell, having looked through a fair bit of research on this over the years, this is one of the most common examples of that phenomenon.

Reasonable studies have come to very different conclusions about which line lengths offer good readability, recommending anywhere from 40 characters or even fewer right up to 100 characters or more. The thing is, those studies usually consider a fairly small number of different line lengths while fixing other layout factors like margin size, text size, leading, and number of columns. However, these other typographical factors are not necessarily consistent from one study to the next, so the results are not directly comparable, nor are conclusions that different line lengths offered the best readability under the specific conditions of each particular study necessarily contradictory. One thing that does seem clear from the relatively few multivariate studies available is that line length is not an independent variable as far as measured readability is concerned. It interacts with those other factors in ways we don’t fully understand yet.

In short, it’s surprisingly unhelpful to propose rules of thumb like “2½ alphabets” or “60 characters” as optimal line lengths, without specifying any context. Those are probably safe choices, in that they have been found to offer good measured readability and to be subjectively pleasing to readers under quite a range of different conditions now. However, that does not mean that they are ideal or optimal and that setting text with a narrower or wider measure would be objectively worse. And if you’re dealing with an audience with more specialised needs, for example the very young or people who have reading difficulties, even those widely suggested rules might turn out to be a bad idea.

Another thing that is clear from the available empirical data is that measured readability is not even close to the same as perceived readability. Readers’ subjective preferences (what they found most comfortable to read) and self-reported performance (what they believe gave the best results in terms of accuracy of retention, reading speed or other objective criteria) may be quite different to their actual, measured performance. Now and then, studies even find a negative correlation: readers actually prefer settings that give measurably worse objective performance. However, it seems much more common that readers express a relatively strong subjective preference for (say) a certain range of line lengths in a study, even though their measured performance didn’t show any significant improvement for those lengths over others that were considered.

TL;DR: While typical guidelines like 60 characters-per-line aren’t absurd and in most conditions would be a reasonable choice, the evidence as a whole shows mixed results, and suggests that while readers may have quite strong subjective preferences, their objective performance doesn’t tend to vary a huge amount until you get to much longer or shorter lines. Describing these mid-length lines as “ideal” or “optimal” is therefore unwarranted.

Amusing irony to finish with: Many of the web-based sources I looked up again while writing this post have terrible line lengths and other typographic settings, blatantly contradicting their own advice. :-)