The reserve list is set in stone, that's the problem. Modern is the only way to solve this eternal format scourge without breaking the reserve list. This keeps revised duals and legacy staples high (ie not screw over legacy investors) while letting common people have a taste of an eternal format. Scg will very likely support all 3 formats IMO. They even have multiple draft opens, so no problem there. I don't think modern is going to hurt legacy's popularity honestly. With people getting into modern first, they'll just slowly drift into legacy since they already have all the non land cards.

Say I have stoneblade in modern, I can later get tundras and forces to play legacy. Modern is not going to hurt legacy IMO. However, if scg drops legacy (which is unlikely since its so popular, you wouldn't hurt your own cash cow for an unproven format right?), legacy will be in huge trouble. It'll likely drop prices to what they were a year ago before the explosion. Ie duals in the 60 ish range.

On another note, the legacy card price poster boy has flatlined/dropped. Wasteland is now $40 on eBay. It was what, 70 once upon a time? Legacy demand is stagnating with all this modern speculation.