As the lore has come down to me, the 1600 cut-off arose because one of the original members liked wearing a Scottish kilt and it was commonly believed at the time that the earliest date at which the kilt appeared was ca. 1600. Now, as it happens, while the earliest date at which the belted plaid can be dated is very roughly ca. 1600, that early SCA member mistakenly believed that it was the modern-style small kilt that could be dated back to 1600.
So ... if this version of events is to be believed (and I heard it from people who were there), the date was not only chosen to cater to a single person's sartorial preferences, but it was chosen based on an entirely erroneous assumption about the history of the garment in question.
You can't get much more Scadian than that.
The sixteen century had modern things like printed literature, handguns, naval warfare, "America" printed on the maps, Protestantism, chocolate, and the contemporary self-evaluation that they were living after the Renaissance - and by extension therefore, after the end of the medieval period.
Preface: This was a time with only 3 kingdoms so one Royal could have a major impact in setting tradition. It spread to the whole Knowne World as other Royals incorporated it into their Kingdom traditions - as it was a right and proper thing.
As I recall being told by many sources, chocolate was declared to be period by a member of the Royals early on in the formation of the SCA (see preface above.) It would then seem that those whose knowledge of the time frames of food becoming common throughout the "western European culture" would make sure that they did not conflict with the law of the SCA as proclaimed by a Queen. One could lose their head, be banished, or be told to take their toys and go home if they did not comply with the Royals wishes - the same as today.
So the cutoff of "period" occurring with the "discovery" of chocolate would seem to be a reasonable lifesaving definition of SCA time frames. Therefore we could define the time under research by the SCA as "from the discovery of chocolate back for a millennium of those living without chocolate, and the activities they did instead of enjoying chocolates such as but not limited to, tournaments, jousts, fairs, dances, classes, et cetera; to acquire authentic or reproduced replicas of chattels representative of said era; and to collect a library of information related to that timeframe."