For now the 717s are not going west.
Also, even if they do it will be very limited. The plan all along has been to have 3 717 bases. ATL was first fallowed quickly by DTW. LAX was being looked at but Network changed their minds on the flying and thus the third and very likely final base is NYC.
I personally wouldn't expect 717s out west for a while, if ever.
yes, it is possible that the decision to base the 717s in NYC is the end of the possibility of them being in LAX or SEA.
What is clear is that LGA after the slot deal became a heavily large RJ hub; the 717s are now taking over some of the Shuttle flights but are also being used for off-season NYC (LGA and JFK) to Florida flights.
Off peak season to Florida is relatively short so those planes will be moved off of those routes and I predict they will free up large RJs. Given the number of large RJs in the system is limited by the pilot contract, large RJs that are freed up have to be deployed elsewhere. It could be at the hubs or it could be to add flights in the western US.
And in the west, unless DL pulls a merger or asset acquisition out of somewhere, it will need to start upgrading existing large RJ flights at both SEA and LAX to mainline... and it is highly likely they will do it. Correct me if I am wrong, but I don't think there is a 320/19 base on the west coast but is at SLC. It is possible that DL will use 319s and 320s and use SLC pilots but the ability to grow the west coast depends on upgrading. The 717 is the smallest mainline aircraft but perhaps there are enough flights that can far beyond the 717 in capacity. LAX-SEA is up to 4 mainline - 2 757s and 2 738s, SEALAS has a couple 738s, and SEASAN for the summer was 3 738s/320s per day.
so, yes, maybe the 717 won't go west but if it doesn't there either will be a lot more 737 west coast flying from LAX and/or a 320 base (which also allows use of the smaller 319). The 717s shorter range does make it a better aircraft for the shorter eastern US flights.
Either way, DL is not thru growing either LAX or SEA and as next summer's schedule is firmed up, it will become more and more indicative of DL's staffing needs on the west coast.
OTOH, DL might be reworking its fleet plan and network depending on what happens with AF this week. I have a feeling it is going to get really nasty.