Is Epic lagging behind? It is a matter of perspective. If you think access to the Land Speeder Storm is critical in making an army Space Marines, then it probably is. Personally, I don't think that's the case. Not that Epic shouldn't include Storms in lists, just that a list doesn't need them to represent Space Marines - even "modern" ones.
I have never understood the view that second edition, third edition, fourth edition and fifth are incompatible with each other. There are aspects that are, yes, but there's no reason Squats can't fight Tau. As new stuff appears, it should be added to the game. Frankly, most armies that existed in second edition haven't become unrecognizable since then. Guard are Guard, Marines are Marines, Orks are Orks. The fact that the current Codex Astartes list is based on Marines as they were in third edition doesn't mean it no longer is capable of representing Space Marines altogether. It means it lacks certain aspects of modern Marines. It also lacks stuff from second edition, and revamping the entire list to accomodate that isn't exactly a pressing goal.
Personally, I think including new stuff in lists is reasonable in theory. However, I also think any benefit of accessibility to new players from offering a Storm Talon is going to be cancelled by the way they have to convert one or proxy it with another model. If accessibility is the goal, then forcing new units that lack models into established lists is going to be a mixed success at best. Putting the Land Raider Crusader or a Thunderhawk variant into the Codex Marine list would be one thing. Putting in the Storm Talon, which has untested rules and lacks a model, simply to accomodate new players because of an as-yet unestablished "iconicness" is another.
On top of this, 40K is currently producing a lot of new models - we used to see one Imperial Armour a year. Now we see two, plus a fair amount of new 40K stuff. And there's a lot of overlap in some of it with both existing Epic stuff and existing 40K stuff. Trying to fit it all in would be a lot of work to little gain.
What I would do is create experimental formations that are independent of a particular list (print them at the end of the compendium). Experimental formations would never be tournament legal, but should be suitable for friendly games. So if we stat up the Spartan, we should also create a more-or-less standard Spartan formation, note that it is an experimental formation for use in friendly games in lists that don't have access to it, then forget about it until someone uses it in a list. Avoids too much meddling with fairly stable lists, and still lets people take new shiny toys with established lists.
EDIT: A new/up-to-date Codex Astartes list wouldn't necessarily be a bad thing, of course. I just don't think Storm Talons, Land Speeder Storms and Thunderfire Cannons are quite enough to hold together a list.
E&CQuote:
- Assault Terminators
- Stormtalon Gunship
- Land Speeder Storm
- Scout Bikes
- Thunderfire Cannon
- Thunderhawk Transporter
- A second Land Raider variant (likely Crusader) would arguably also make the cut.
Er...Assault Terminators and Scout Bikes both existed when the current list was created.