"X-Men: First Class" was essentially a reboot that didn't require individual franchises leading up to its release.
They could pull the JL film off without preceding franchises.
Let's not forget that "Avengers" was great...because it was a great film on its own merit. If they'd done all the preceding groundwork and made a stink nugget that work would've been in vain.
It's like when I'm right I'm right, when I'm wrong I could been right, so I'm still right cause I coulda been wrong.