[Your estimate of building only 5500A gaining 4000A is way off, you will be building a lot closer to 8000A.]
It is not way off.
Scenario B:
2012-12-31 00:47:30 PS 14 (#168) PANLV 1 (#155) PANLV 5159A (9246A)
2012-12-31 00:44:08 PS 1 (#155) PANLV 14 (#168) PANLV 4962A (9069A)
http://www.eestats.com/ffa/oldcountry/405/155
First attacker gains 9069 acres and build 4962 Acres.
2nd attacker gains 9246A acres and builds 5159 Acres.
1st Attacker nets 3910 Acres and had to build 4962.
2nd Attacker nets 4284 acres and had to build 5159.
Dictators with the similar NW land trading have to build about 125% of the net acres gained. You lose about 1.5% of your Construction Sites, but you do not need to rebuild at this point because your BPT is already past optimal for the future acres built.
In summary it would cost you 1 attack with likely 4 spy ops so 6 turns and a gain of 4000 acres would take 50 turns to build. It would destroy 1M turrets (on the high side). Costing you $125M (also on the high side since some indys are selling turrets for $95). The $125M is going to take 5 turns of cashing to recover depending on your tech levels. the extra 1k acres built will cost you another $166M or so, so probably 6 turns cashing.
Landtrading gain of 4000 acres nets to 71 turns to build and recoup investment.
Scenario A: Bottomfeeding with 100BPT for 4k land will take 6 attacks (5 turns/attack on average - this is generous, I fail a ton of ops these days). 30+40 turns, so you gain 4k land in 70 turns
The hidden benefit of Scenario A is that you may not be building 4k land if you grab targets of the same strategy.
The hidden benefit of Scenario B is that you are doing your cashing and building at 4k acres more already. So your cash/turn may be increased - depending on tech% decrease.
So in all I am not sure I would be landtrading if I were you, but I wasn't a fool to think about it. And you weren't doing the idea justice in your analysis.
"But this gets worse the more you trade, and you won't have money to maintain your tech base and you will fall behind your non-trading competitors."
No. It improves the more you trade because you gain land faster. The more acres you have the more superior landtrading is to bottomfeeding.