Spielberg has historically been very good with tight production schedules. It's the script-writing phase that needs time to percolate, but two years should be more than enough if they're actually working on it.
Trying to liken it to how the previous film came together probably isn't an apples-to-apples comparison. The development of Indiana Jones 4 seemed to be less about scripts being rejected for quality reasons than being rejected because Spielberg and Lucas kept shooting down the other's story idea. if you break down the development timeline, each individual screenwriter was handed a story outline and had a pretty reasonable period of 12-18 months on the project during which they turned in a couple of drafts, usually under the direction of
one of the Beards.
With the exception of the first incarnation (which was written by Stuart then passed to Boam), each screenwriter was association with a single incarnation of Indy 4's story. When Boam turned in his final draft that satisfied Lucas, Spielberg did not turn it down because he thought it sucked, but because he didn't think the approach was valid after Independence Day. It's worth noting that Stuart and Boam worked pretty much exclusively under Lucas (possibly because Spielberg still had the "I'll just be a producer" attitude toward the project during that initial phase).
A few years pass, then Lucas comes up with the Chariots of the Gods approach. Story conferences are held, Lucas and Spielberg hire Darabont. Darabont works on his drafts for a year or so almost exclusively under Spielberg (possibly because Lucas was knee-deep in the Star Wars prequels), and turns in a final draft to Spielberg's satisfaction. Lucas shoots it down for reasons unknown.
The project is reset again. Jeff Nathanson is hired and more story conferences are held. Unlike his predecessors, Nathanson toggles between Spielberg and Lucas when he writes his drafts, which Lucas tellingly
admits caused the screenwriter to be caught between him and Spielberg and probably doomed his version.
Project reset again. Koepp is hired. He doesn't want a repeat of what happened to Nathanson, so he insists on working directly under Spielberg. A year and a few drafts later, the script finally get everyone's approval. Was it a matter of quality, or was everyone's resistance just knackered by attrition at that point? At any rate, that's how things ended up for the fourth movie.
When you look at that pattern, you don't so much get the impression that the writers were doing poor work but rather that Lucas and Spielberg kept disagreeing with each other's ideas - the screenwriters were just the operatives hired to flesh those ideas. With Lucas seemingly not involved with Indiana Jones 5 in a creative capacity, the project shouldn't be dealing with that issue. I think with Indy 5, it's simply a matter of whether the project is actually getting attention. If Spielberg and Koepp have been really focusing on this project, they should already have something solid that they are polishing to a shine. If not...