It seems that there is indeed a strong "climate-case" in favour of wetlands: Below follows a bullet-list of several facts and arguments, which among other things undermine the premises of my question (to a certain degree):
Currently, 3% of the land surface is covered by peat lands. But it is estimated that up to 1/3 of all terrestrial carbon is stored in those peat lands, giving a big edge in carbon sequestration/m2 over any other type of vegetation. Although these peat lands convert some of their intake of CO2 in the more potent greenhouse gas CH4, it is clear that the remedy is certainly not to 'drain the swamp' in a short time-window in a way where all the carbon is injected in the atmosphere. Since the latter is exactly the threat posed to many wetlands nowadays, there is clear and in some cases perhaps decisive motivation and urgency for the conservation and protection of existing wetlands.
Organic methanogenesis appears strongly temperature-dependent: satellite data show the tropical regions as the main global sources. Laboratory experiments roughly confirm that picture, although many questions remain unanswered and methanogenesis seems to depend on a wide range of variables. In any case, in temperate and boreal regions the wetland-contribution in CH4-emission often pales in comparison to agricultural and industrial sources (in Flanders, agriculture contributes 20-fold the CH4 contribution from rivers and wetlands). Hence, in densely-populated temperate and boreal regions, the CH4-emission-argument may be moot since there are more opportune alternative options for climate-change mitigation, with less collateral consequences (wetlands provide biodiversity, hydrological stability etc).
The argument that wetlands saturate and stop net intake of carbon may be partly false and largely irrelevant: A mature peatland may reach a depth varying from 2 to 20 meters, while a peatland's annual growth is typically of order 1 mm/ year. With some audacity, I propose to infer that a peatland then needs a time of order 10m/(10-3 m/year)= 10000 years to grow to maturity. During that time, it did absorb carbon to grow. Since 10000 years dwarfs the time-scale in which humanity is to overcome and adapt to climate change, the saturation-argument against reinstating or re-valuation of peatlands is irrelevant. The saturation-argument might be weakened even more if one includes in the calculation the depth of the coal-seam which is often found below a mature peatland.