Mercs should be given another chance, they never got the chance to be as popular as the damnrebels, even though they seserve it more.
Fading will probably be in Planeshift mainly beacuse the storyline sounds like it calls for it (it's the reverse of Nemesis basically, where things are now going to fade OUT of Rath, as opposed to fading in from Dominaria.
I see no reason they can't print more Rebels and Mercenaries, but it won't happen in Invasion. Maybe they'll go back to Mercadia after that (please?), as we never see the end result there and the Cateran Guild is hardly out of the picture. After all, what else could happen in Dominaria and Rath after Apocalypse(dear god not more Phyrexian techno-magic idiocy. The technology vs. nature thing is getting old, the Phyrexians are really out of place in the setting, but then I just hate the fact that artifacts have gone from being swords and amulets to such insane things as Phyrexian Colossuses and Predator, Flagships. Oh who cares, the airships can stay, but drop the rest of the whole thing and go back to actual fantasy stuff)