[jfengel]

Most of the cognates are direct, but not quite all. The verb in "icumen" is in fact cognate to "come", but the prefix and suffix are false cognates.

The verb is actually in the present perfect, not the progressive, so it means more like "has come" rather than "is coming". It's much more cognate in modern German: "gekommen", which still means "has come".

Still, you're generally correct: you can understand a ton of Middle English with nothing more than modernizing the spelling. Here's Canterbury Tales with modern spelling (and notes for the words that genuinely don't translate):

http://academic.brooklyn.cuny.edu/webcore/murphy/canterbury/

Old English is very foreign, but a few centuries after the Norman Invasion, the gap from Chaucer to Shakespeare to today is all much smaller. Chaucer is "Middle English" and Shakespeare is "Early Modern", but those changes are more about sound shifts than actual changes in language. (You'd never understand spoken Chaucer without a serious lesson, but authentic Shakespeare requires just a little close listening.)