All the updates you talked about - new classes, new gear, new shinies - those things are fairly easy to make. You can throw a team of graphic designers at it and pay their salaries pretty easily based on how many shinies they designed sell. If the code is done properly, which from the way you've described it sounds like the underlying game engine is fairly robust, then creating a new class and balancing it is easier than you'd believe. It's just numbers and graphics and then running simulations and then tweaking those numbers and graphics until the stuff you're putting in is optimized.
A deep and compelling storyline with cinematic cutscenes requires all of the above (graphic designers, game engineers) as well as a team of writers and a CG artists, doubling the number of people involved. In IT projects, when you toss more bodies into any given project, the amount of time required to complete the project escalates on a logarithmic scale. Putting out new armor every two weeks is doable, putting out a new chunk of storyline every two weeks is considerably more difficult and more expensive.
Could a game designed from the ground up as F2P have a deep and compelling storyline? It's entirely possible. Have I seen it happen yet? No. (SWTOR was not designed as F2P.)
This is a miscomception that i have heard around here a few times, balancing is easy is just numbers etc etc, yet two of the most successful companys have had always problems as far as balancing classes, one is blizzard with it's PVP, and the other one is Legue of Legends, entierly different companys in different genres, with the same problem, i would think if it was as easy as tweaking a number here and there, they would have ridden themselfs of such problems a long time ago, yet every new update from both companys brings fixes and buffs or nerfs to their content in order to balance classes. They are professional programers with what i would think is an infinite more experience than some random posters in some board, and they have not been able to fix this "Easy" balancing problem.... Hmm! Maybe it is not as easy as adding one number here and subtracting another there ?
As far as a deep and compelling storyline, that is entierly subjective, some people claim FFVII has the deepest most compelling storyline, to me is utter garbage, as far as storyline is concerned i can name from the top of my head 5 games with better storylines: Xenogears, Suikoden II, Final Fantasy Tactics, Final Fantasy VI, Chrono Cross/Trigger, Arc the lad Series. Also take into account that the golden age of RPG'S was the PSX era, where we had very few CG and cutscenes out of anime/movies, if a game had some they where few and spread around the game, unlike today where 50% of the games are CG and the other 50 is actually playing the game (Xenosaga series is blatant offender of this) this notion that game in order to deliver a compelling storyline needs CG and "AWESOME GRAPHIPCS" is silly and stupid, FFXIV is the MMO with the most pretty graphics, its storyline compared to it's pretty graphics is utter garbage as of today, swotors cartoony engine delivers more storyline per mission that FFXIV would ever hope to acomplish today (Hoping it would change in ARR) ever since the PS2 era, RPGS have seen a drastic drop in quality, because companys figure out, that if you put some shiny graphics and some CG people will buy a title ( XIII is proof of this )
Really name me one PS2 Game or PS3 or Xbox/360 RPG that was better than a top 5-10 RPG from the PSX Era :) and i will name you 3 games from the psp or nds that are better than it, because really good RPGS are no longer in consoles, they are in handhelds, all you get in consoles are games worth a trailer and once you pop the cd on the system and play for 3 hours, you are bored to death with them, or already know whats gonna happen.