[BScout]

It's been mentioned the S6-M10 will be supported as long as the S6-M40

The displays are the same cost as for the S6-M40 (because they are the same displays) for both M10 and S4

The S6-M10+ option dings you with $3k license charge to support the displays but then you add the price of the first new display on top to get around $10k. This is in the press release.

An S6-M40 is not the same as the S6-M10+ because the M10+ still has the same (current) limited expandability (also found in the press release for the + option since it's mentioned with how many displays can be added.)

Highly doubtful the S6 will be dropped anytime soon. Bigger facilities are just putting them in now still and it's still in its adolescence. There are so many features that are being added still (and stuff from the Icons still aren't there.)
Plus Avid has a guarantee of prolonged support for the surfaces after EOL by at least 5 years (it may be 10 years after EOL)