Eh? I don't understand your argument. Polders have higher base yields than farms, and buff farms on a 1:6 ratio (i.e. 1 polder can influence up to 6 farms) instead of a 2:1 ratio (i.e. 2 farms can influence 1 other farm). So why would you ever not build a polder, especially if you can then surround it with a mix of farms and trading posts? Even considering farm adjacency the Polders would give you more base yields on nearby farms than the adjacency possibly could. If you covered a river, end-to-end, with polders, all farms adjacent to them would have anywhere from +2 to +5 additional food (depending on the shape of the river), not to mention any adjacency bonuses you could create for those farms based on river-shape. Throw in a trading post or two if desired, and you've got a solid set of tiles that really boost the power of farms.
NOW, that said, I'm perfectly fine with stepping back and, to further distinguish Polders from Terrace Farms, making Polders focus exclusively on villages/towns. The bonus would need to be stronger than +1 gold, though.
G