I think they might have ummed about it right at the start, but apparently they made an early decision not to include it. Not because it wouldn't be easy to shoot the chain rising (it would have been) but because the scenes of the ships crashing into the chain and the wildfire jumping from ship to ship would have required dozens of CGI shots, whilst the 'fuel air bomb' explosion only required a handful. With each CGI shot costing thousands, they had to ration them.

As for the scope, the producers knew this going in. Their rationale is probably that they can do one big battle per season (aside from the first season, when the immense costs of building the sets ate into their battle budget) and this will - more or less - work. So we get the skirmish with the wildlings attacking Castle Black from the south in Season 3, the full-scale assault on the Wall in Season 4, and they might then be able to separate the battle at Meereen and the battle at Winterfell between the end of one season (probably 5) and the start of the next (6) (and have the battle at Storm's End happen offscreen).

If the show's popularity continues to rise, future budget increases are also possible. The show may also follow in the footsteps of the likes of BSG and BABYLON 5 and create an in-house CGI department rather than outsourcing to external companies. This will give them immensely more time and money to do a lot more CGI in later seasons to handle more battles and more appearances by the dragons. The dragons should already be the size of dogs in Season 3, and be of a rideable size by Season 5 or 6 at the latest, so they are going to have to come up with some ideas on that front.