The duration doesn't get me so much as the release schedule.
Honestly, I don't know what the holdup is on releasing modules. Whether it's lack of modules, lack of art, lack of editing, or just failure to coordinate, all I know is this:
I tried to run Season 2 of Shadowrun Missions. It failed, because my players got bored with the lack of new missions to play.
I tried to run Season 3 of Shadowrun Missions. It failed, because my players got bored with the lack of new missions to play.
I tried to run Season 4 of Shadowrun Missions. It failed, because my players got bored with the lack of new missions to play.
I just had a major gaming convention in town, where I wanted to kick off Season 5 of Shadowrun Missions. The first module, which was supposed to be released after GenCon, instead was not released in time to run at the convention. So already Season 5 in town has missed out on players.
That's where the situation is. A release schedule of one module a month, for 24 months, will not sustain a regular campaign, at least in my experience.
Is it a lack of modules? I'll step up and write some, if need be. I've done it before. A lack of art? There I cannot help you. Editing? There I can. Coordination? I have nothing BUT time.
Running previous Seasons might be an option were it not for the 'hard reboot' at the beginning of Season 5. Having the players go from Denver to Seattle to Chicago gets a little confusing, not to mention costly for the PCs.
"But Deacon! You can run your own campaign in the interim!" Yeah, right. My players want to run the official campaign. Not my homebrew. I need more Missions, to satisfy players. Otherwise Season 5 will fail just like before.